At least four people were killed and hundreds wounded, according to witnesses and state TV.
Abdel Jalil Khalil, a leading Shia politician, said troops had fanned out across Bahrain, cutting off streets and arresting or firing at people who tried to cross.
He added private homes and funeral parlours were receiving casualties as troops had surrounded hospitals and were not letting people in.
"This is a war of annihilation," Mr Khalil told the Reuters news agency, "This does not happen even in wars and this is not acceptable."
Neighbouring Iran has denounced the military action as "foul and "doomed". More + video...