Troops from a number of Gulf states, including Saudi Arabia and the UAE, have arrived in Bahrain at the request of the kingdom, officials say.
It comes a day after the worst violence since seven anti-government protesters were killed in clashes with security forces last month.
Dozens of people were injured on Sunday as protesters pushed back police and barricaded roads.
Bahrain's opposition said the foreign forces amounted to an occupation.
But the kingdom's authorities urged the population to "co-operate fully and to welcome" the troops, the AFP news agency reports.
A Saudi official said about 1,000 Saudi Arabian troops arrived in Bahrain early on Monday, and later the UAE said it had sent some 500 police officers.
Witnesses told the Reuters news agency that about 150 Saudi Arabian armoured troop carriers plus other vehicles entered Bahrain on the causeway that links the two kingdoms.
The Saudi government said in a statement that it "has answered a request by Bahrain for support", according to the Saudi Spa state-run news agency.
The troops are part of a deployment by the Gulf Co-operation Council (GCC), a six-nation regional grouping which includes Bahrain, Saudi Arabia, Kuwait, Oman, Qatar and the United Arab Emirates.
It is believed they are intended to guard key facilities such as oil and gas installations and financial institutions.
The US said it was aware of the deployment.
White House spokesman Tommy Vietor said: "We urge our GCC partners to show restraint and respect the rights of the people of Bahrain, and to act in a way that supports dialogue instead of undermining it."
The US Navy bases its Fifth Fleet in Bahrain.
The British and Australian governments have warned against all travel to the kingdom.
Bahrain's Shia majority has long complained of discrimination at the hands of the Sunni ruling elite, but large-scale protests broke out last month after the presidents of Egypt and Tunisia were toppled in uprisings.
On Monday, protesters continued their occupation of Pearl Square, near Manama's financial district, and set up roadblocks around the area.
In a statement issued before the arrival of the GCC troops was confirmed, the Shia-led opposition said: "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."
King Hamad bin Issa al-Khalifah has offered dialogue with the protesters, but they have refused, saying they want the government to step down.
Most of the opposition and protesters have said they do not want to overthrow the monarchy, but want the ruling family to give up most of its powers to the elected parliament.
Some, however, have said they want a republic.
The intervention from Bahrain's predominantly Sunni neighbours may deepen the rift between Shia and Sunni Muslims in Bahrain and beyond, says the BBC's Middle East analyst Magdi Abdelhadi.
Saudi Arabia, which has problems with its own Shia minority, has already clamped down on Shia democracy activists, our analyst says.