LONDON — Google, Facebook, TikTok and other Big Tech companies operating in Europe are facing one of the most far-reaching efforts to clean up what people encounter online.

The first phase of the European Union’s groundbreaking new digital rules will take effect this week. The Digital Services Act is part of a suite of tech-focused regulations crafted by the 27-nation bloc — long a global leader in cracking down on tech giants.

The DSA, which the biggest platforms must start following Friday, is designed to keep users safe online and stop the spread of harmful content that’s either illegal or violates a platform’s terms of service, such as promotion of genocide or anorexia. It also looks to protect Europeans’ fundamental rights like privacy and free speech.

Some online platforms, which could face billions in fines if they don’t comply, have already started making changes.

Here’s a look at what’s happening this week:

WHICH PLATFORMS ARE AFFECTED?

So far, 19. They include eight social media platforms: Facebook, TikTok, Twitter, YouTube, Instagram, LinkedIn, Pinterest and Snapchat.

There are five online marketplaces: Amazon, Booking.com, China’s Alibaba AliExpress and Germany’s Zalando.

Mobile app stores Google Play and Apple’s App Store are subject, as are Google’s Search and Microsoft’s Bing search engine.

Google Maps and Wikipedia round out the list.

WHAT ABOUT OTHER ONLINE COMPANIES?

The EU’s list is based on numbers submitted by the platforms. Those with 45 million or more users — or 10% of the EU’s population — will face the DSA’s highest level of regulation.

Brussels insiders, however, have pointed to some notable omissions from the EU’s list, like eBay, Airbnb, Netflix and even PornHub. The list isn’t definitive, and it’s possible other platforms may be added later on.

Any business providing digital services to Europeans will eventually have to comply with the DSA. They will face fewer obligations than the biggest platforms, however, and have another six months before they must fall in line.

Citing uncertainty over the new rules, Meta Platforms has held off launching its Twitter rival, Threads, in the EU.

WHAT’S CHANGING?

Platforms have started rolling out new ways for European users to flag illegal online content and dodgy products, which companies will be obligated to take down quickly and objectively.

Amazon opened a new channel for reporting suspected illegal products and is providing more information about third-party merchants.

TikTok gave users an “additional reporting option” for content, including advertising, that they believe is illegal. Categories such as hate speech and harassment, suicide and self-harm, misinformation or frauds and scams, will help them pinpoint the problem.

Then, a “new dedicated team of moderators and legal specialists” will determine whether flagged content either violates its policies or is unlawful and should be taken down, according to the app from Chinese parent company ByteDance.

TikTok says the reason for a takedown will explained to the person who posted the material and the one who flagged it, and decisions can be appealed.

TikTok users can turn off systems that recommend videos based on what a user has previously viewed. Such systems have been blamed for leading social media users to increasingly extreme posts. If personalized recommendations are turned off, TikTok’s feeds will instead suggest videos to European users based on what’s popular in their area and around the world.

The DSA prohibits targeting vulnerable categories of people, including children, with ads.

Snapchat said advertisers won’t be able to use personalization and optimization tools for teens in the EU and U.K. Snapchat users who are 18 and older also would get more transparency and control over ads they see, including “details and insight” on why they’re shown specific ads.

TikTok made similar changes, stopping users 13 to 17 from getting personalized ads “based on their activities on or off TikTok.”

IS THERE PUSHBACK?

Zalando, a German online fashion retailer, has filed a legal challenge over its inclusion on the DSA’s list of the largest online platforms, arguing that it’s being treated unfairly.

Nevertheless, Zalando is launching content flagging systems for its website even though there’s little risk of illegal material showing up among its highly curated collection of clothes, bags and shoes.

The company has supported the DSA, said Aurelie Caulier, Zalando’s head of public affairs for the EU.

“It will bring loads of positive changes” for consumers, she said. But “generally, Zalando doesn’t have systemic risk (that other platforms pose). So that’s why we don’t think we fit in that category.”

Amazon has filed a similar case with a top EU court.

WHAT HAPPENS IF COMPANIES DON’T FOLLOW THE RULES?

Officials have warned tech companies that violations could bring fines worth up to 6% of their global revenue — which could amount to billions — or even a ban from the EU. But don’t expect penalties to come right away for individual breaches, such as failing to take down a specific video promoting hate speech.

Instead, the DSA is more about whether tech companies have the right processes in place to reduce the harm that their algorithm-based recommendation systems can inflict on users. Essentially, they’ll have to let the European Commission, the EU’s executive arm and top digital enforcer, look under the hood to see how their algorithms work.

EU officials “are concerned with user behavior on the one hand, like bullying and spreading illegal content, but they’re also concerned about the way that platforms work and how they contribute to the negative effects,” said Sally Broughton Micova, an associate professor at the University of East Anglia.

That includes looking at how the platforms work with digital advertising systems, which could be used to profile users for harmful material like disinformation, or how their livestreaming systems function, which could be used to instantly spread terrorist content, said Broughton Micova, who’s also academic co-director at the Centre on Regulation in Europe, a Brussels-based think tank.

Under the rules, the biggest platforms will have to identify and assess potential systemic risks and whether they’re doing enough to reduce them. These risk assessments are due by the end of August and then they will be independently audited.

The audits are expected to be the main tool to verify compliance — though the EU’s plan has faced criticism for lacking details that leave it unclear how the process will work.

WHAT ABOUT THE REST OF THE WORLD?

Europe’s changes could have global impact. Wikipedia is tweaking some policies and modifying its terms of service to provide more information on “problematic users and content.” Those alterations won’t be limited to Europe, said the nonprofit Wikimedia Foundation, which hosts the community-powered encyclopedia.

“The rules and processes that govern Wikimedia projects worldwide, including any changes in response to the DSA, are as universal as possible. This means that changes to our Terms of Use and Office Actions Policy will be implemented globally,” it said in a statement.

It’s going to be hard for tech companies to limit DSA-related changes, said Broughton Micova, adding that digital ad networks aren’t isolated to Europe and that social media influencers can have global reach.

The regulations are “dealing with multichannel networks that operate globally. So there is going to be a ripple effect once you have kind of mitigations that get taken into place,” she said.