A law has come into effect in Germany requiring social media companies to remove "obviously illegal" posts or pay fines of up to 50m euro (£44.3m).
Companies with more than two million German users will have 24 hours to remove posts containing hate speech or other criminal material.
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.
Theresa May told the United Nations General Assembly last month that tech giants must go "further and faster" to remove extremist content. Along with France and Italy, she is calling for a target of one to two hours for censors to remove illegal material.
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".
It comes into effect one week after the European Commission published guidelines urging social networks to be more proactive in preventing and removing hate speech.