Social media companies in Germany are now required to remove “obviously illegal” posts or pay fines of up to €50m.
Under the new rules, companies with more than two million German users will have 24 hours to remove posts containing hate speech or other criminal material.
As reported by the BBC, the law is among the toughest of its kind in the world.
Critics have said the law’s tight time limits are unrealistic and may lead to accidental censorship.
German MPs voted for the Netzwerkdurchsetzungsgesetz (NetzDG) law in June after a series of high-profile cases of fake news and hate speech being spread on social media sites in the country.
Companies such as Facebook, YouTube and Twitter will have more time to evaluate reports of speech which is not obviously criminal – seven days instead of one.
David Kaye, the UN’s Special Rapporteur on Freedom of Expression, said the law “raises concerns about freedom of expression”.