Protests are taking place in Moscow as Ukraine's Crimea region is getting ready for a vote on whether to become part of Russia.

Tens of thousands marched to protest its intervention in Ukraine on the eve of the vote that the U.S. and its European allies say is illegal and unconstitutional.  

U.S. officials expect Crimean voters will choose to become part of Russia.

They also say if Russia doesn't make an immediate effort to negotiate with Ukraine on resolving the crisis, stiff sanctions against Russia could happen as soon as Monday.