The intervention reportedly comes after repeated calls by the Bahraini government
for dialogue went unanswered by the opposition.
But in a statement, an alliance of seven opposition groups said the foreign military intervention "violates international conventions" and was considered to be an occupation.
"We consider the arrival of any soldier, or military vehicle, into Bahraini territory an overt occupation of the kingdom of Bahrain and a conspiracy against the unarmed people of Bahrain," the statement said.
The opposition called for an emergency meeting of the UN Security Council, and appealed for international intervention.
On Monday, pro-democracy protesters in the Gulf state, which has been ruled by a Sunni dynasty for more than 200 years, took over the central business district of Manama, the capital.
Helicopters hovered over the Financial Harbour business complex, a day after more than 200 people were injured there in clashes between riot police and demonstrators.
Sunday was the worst day of violence in Bahrain since seven people were killed at the start of unrest a month ago.