The Facebook page in Slovakia called Som z dediny, which means “I’m from the village,” trumpeted a debunked Russian claim last month that Ukraine’s president had secretly purchased a vacation home in Egypt under his mother-in-law’s name.

A post on Telegram — later recycled on Instagram and other sites — suggested that a parliamentary candidate in the country’s coming election had died from a Covid vaccine, though he remains very much alive. A far-right leader posted on Facebook a photograph of refugees in Slovakia doctored to include an African man brandishing a machete.

As Slovakia heads toward an election on Saturday, the country has been inundated with disinformation and other harmful content on social media sites. What is different now is a new European Union law that could force the world’s social media platforms to do more to fight it — or else face fines of up to 6 percent of a company’s revenue.

The law, the Digital Services Act, is intended to force social media giants to adopt new policies and practices to address accusations that they routinely host — and, through their algorithms, popularize — corrosive content. If the measure is successful, as officials and experts hope, its effects could extend far beyond Europe, changing company policies in the United States and elsewhere.

The law, years of painstaking bureaucracy in the making, reflects a growing alarm in European capitals that the unfettered flow of disinformation online — much of it fueled by Russia and other foreign adversaries — threatens to erode the democratic governance at the core of the European Union’s values.

Europe’s effort sharply contrasts with the fight against disinformation in the United States, which has become mired in political and legal debates over what steps, if any, the government may take in shaping what the platforms allow on their sites.

A federal appeals court ruled this month that the Biden administration had very likely violated the First Amendment guarantee of free speech by urging social media companies to remove content.

Europe’s new law has already set the stage for a clash with Elon Musk, the owner of X, formerly known as Twitter. Mr. Musk withdrew from a voluntary code of conduct this year but must comply with the new law — at least within the European Union’s market of nearly 450 million people.

“You can run but you can’t hide,” Thierry Breton, the European commissioner who oversees the bloc’s internal market, warned on the social network shortly after Mr. Musk’s withdrawal.

The election in Slovakia, the first in Europe since the law went into effect last month, will be an early test of the law’s impact. Other elections loom in Luxembourg and Poland next month, while the bloc’s 27 member states will vote next year for members of the European Parliament in the face of what officials have described as sustained influence operations by Russia and others.

While the law’s intentions are sweeping, enforcing the behavior of some of the world’s richest and most powerful companies remains a daunting challenge.

That task is even more difficult for policing disinformation on social media, where anybody can post their views and perceptions of truth are often skewed by politics. Regulators would have to prove a platform had systemic problems that caused harm, an untested area of law that could ultimately lead to years of litigation.

Enforcement of the European Union’s landmark data privacy law, known as the General Data Protection Regulation and adopted in 2018, has been slow and cumbersome, though regulators in May imposed the harshest penalty yet, fining Meta 1.2 billion euros, or $1.3 billion. (Meta has appealed.)

Dominika Hajdu, the director of the Center for Democracy and Resilience at Globsec, a research organization in Slovakia’s capital, Bratislava, said only the prospect of fines would force platforms to do more in a unified but diverse market with many smaller nations and languages.

“It actually requires dedicating quite a large sum of resources, you know, enlarging the teams that would be responsible for a given country,” she said. “It requires energy, staffing that the social media platforms will have to do for every country. And this is something they are reluctant to do unless there is a potential financial cost to it.”

The law, as of now, applies to 19 sites with more than 45 million users, including the major social media companies, shopping sites like Apple and Amazon, and the search engines Google and Bing.

The law defines broad categories of illegal or harmful content, not specific themes or topics. It obliges the companies to, among other things, provide greater protections to users, giving them more information about algorithms that recommend content and allowing them to opt out, and ending advertising targeted at children.

It also requires them to submit independent audits and to make public decisions on removing content and other data — steps that experts say would help combat the problem.

Mr. Breton, in a written reply to questions, said he had discussed the new law with executives from Meta, TikTok, Alphabet and X, and specifically mentioned the risks posed by Slovakia’s election.

“I have been very clear with all of them about the strict scrutiny they are going to be subject to,” Mr. Breton said.

In what officials and experts described as a warning shot to the platforms, the European Commission also released a damning report that studied the spread of Russian disinformation on major social media sites in the year after Russia invaded Ukraine in February 2022.

“It clearly shows that tech companies’ efforts were insufficient,” said Felix Kartte, the E.U. director with Reset, the nonprofit research group that prepared the report.

Engagements with Kremlin-aligned content since the war began rose marginally on Facebook and Instagram, both owned by Meta, but jumped nearly 90 percent on YouTube and more than doubled on TikTok.

“Online platforms have supercharged the Kremlin’s ability to wage information war, and thereby caused new risks for public safety, fundamental rights and civic discourse in the European Union,” the report said.

Meta and TikTok declined to comment on the enactment of the new law. X did not respond to a request. Ivy Choi, a spokeswoman for YouTube, said that the company was working closely with the Europeans and that the report’s findings were inconclusive. In June, YouTube removed 14 channels that were part of “coordinated influence operations linked to Slovakia.”

Nick Clegg, president of global affairs at Meta, said in a blog post last month that the company welcomed “greater clarity on the roles and responsibilities of online platforms” but also hinted at what some saw as the new law’s limits.

“It is right to seek to hold large platforms like ours to account through things like reporting and auditing, rather than attempting to micromanage individual pieces of content,” he wrote.

Slovakia, with fewer than six million people, has become a focus not just because of its election on Saturday. The country has become fertile ground for Russian influence because of historical ties. Now it faces what its president, Zuzana Caputova, described as a concerted disinformation campaign.

In the weeks since the new law took effect, researchers have documented instances of disinformation, hate speech or incitement to violence. Many stem from pro-Kremlin accounts, but more are homegrown, according to Reset.

They have included a vulgar threat on Instagram directed at a former defense minister, Jaroslav Nad. The false accusation on Facebook about the Ukrainian president’s buying luxury property in Egypt included a vitriolic comment typical of the hostility in Slovakia that the war has stoked among some. “He only needs a bullet in the head and the war will be over,” it said. Posts in Slovak that violate company policies, Reset’s researchers said, had been seen at least 530,000 times in two weeks after the law went into effect.

Although Slovakia joined NATO in 2004 and has been a staunch supporter and arms supplier for Ukraine since the Russian invasion, the current front-runner is SMER, a party headed by Robert Fico, a former prime minister who now criticizes the alliance and punitive steps against Russia.

Facebook shut down the account of one of SMER’s candidates, Lubos Blaha, in 2022 for spreading disinformation about Covid. Known for inflammatory comments about Europe, NATO and L.G.B.T.Q. issues, Mr. Blaha remains active in Telegram posts, which SMER reposts on its Facebook page, effectively circumventing the ban.

Jan Zilinsky, a social scientist from Slovakia who studies the use of social media at the Technical University of Munich in Germany, said the law was a step in the right direction.

“Content moderation is a hard problem, and platforms definitely have responsibilities,” he said, “but so do the political elites and candidates.”