KIEV, Nov 24 (Reuters) - Tens of thousands of Ukrainians, bearing EU flags and chanting "Down with the gang!", marched through the capital on Sunday in a pro-Europe rally denouncing President Viktor Yanukovich's U-turn in policy back towards Russia.

In the biggest street demonstrations in Kiev since the Orange Revolution nine years ago, opposition leaders called on Yanukovich to go ahead with the signing of a landmark agreement with the European Union on Nov. 29 as originally planned.

Prime Minister Mykola Azarov's government, which stunned European leaders on Thursday by announcing it was suspending preparations for signing a key pact with the EU after years of negotiations and reviving trade and economic talks with Russia, should step down, they said.