MOSCOW — Major Internet sites and human rights advocates sharply criticized a proposed law that would grant the Russian government broad new powers to restrict Web content, ostensibly to protect children from pornography and other harmful material. Critics said the law could quickly lead to repression of speech and a restrictive firewall like the one in China.

Wikipedia, the free online encyclopedia, shut its Russian Web site on Tuesday to protest the proposed measure, and instead posted a large warning on its home page: “Imagine a world without free knowledge.” The notice said the proposed law “can lead to the creation of extrajudicial censorship of the Internet in Russia, including the closure of access to Wikipedia.”

The new measure is part of a wider effort by the Russian authorities to crack down on the opposition since President Vladimir V. Putin’s inauguration in May. They have adopted a law sharply increasing financial penalties on protesters who take part in unsanctioned rallies, begun criminal investigations into several political opposition leaders and considered a plan to require nongovernmental organizations receiving foreign aid to register as foreign agents and face additional auditing and reporting requirements.

With television networks in Russia — and most newspapers and other media outlets — under tight government control, the Internet has emerged as the primary medium for political discourse. Citizens using cellphone cameras documented fraud in last December’s parliamentary elections, then posted video to YouTube and other sites. Organizers of the huge anti-government protests that followed turned to Facebook and other social media to draw tens of thousands of demonstrators on to the streets of Moscow.