Armored troops rolled into Bahrain from neighboring Saudi Arabia on Monday to help restore order in the strategic Gulf kingdom, where pro-democracy demonstrators have shut down the financial center.
Thousands of mainly Shiite protesters occupied Manama's business district, turning the regional banking hub into a ghost town as they pressed their calls for democratic change from the Sunni Muslim monarchy.
The Saudi government said it had responded to a call for help from its neighbor as Saudi-led forces from the Gulf countries' joint Peninsula Shield Force crossed the causeway separating the two countries.
"The council of ministers has confirmed that it has answered a request by Bahrain for support," the Saudi government said in a statement carried by the SPA state news agency.
It said that under an agreement of the six-country Gulf Cooperation Council (GCC), "any harm done to the security of a member state is considered a harm done to the security of all members."
The exact make-up of the force was not known, but the United Arab Emirates also confirmed it was participating in the operation. The GCC groups Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the UAE.
Television footage showed convoys of unmarked, desert-brown colored armored vehicles crossing the causeway from Saudi's Eastern Province into Bahrain, the home of the US Fifth Fleet.
The Shiite-led opposition alliance said any foreign force would be treated as an invading army.
"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," said an opposition statement.
Helicopters buzzed overhead as protesters blocked access roads to the Financial Harbor 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 the tiny Gulf kingdom since seven people were killed at the start of anti-regime unrest a month ago.
Shiite-majority Bahrain has transformed itself into a regional financial center as it seeks to diversify its economy away from dependence on diminishing oil revenues.
But to many of the country's disenfranchised Shiites the banking district is a symbol of corruption, wealth and privilege.
Police appeared to have deserted the area, while shopping malls and office towers were closed.
Protesters persisted with a sit-in at nearby Pearl Square, where activists were readying for a showdown with the security forces.
Most workers seemed to be following a trade union call for a general strike to protest against violence by the security forces.
The Saudi intervention comes two days after US Defence Secretary Robert Gates visited Manama and urged King Hamad to undertake rapid and significant democratic reform, not just "baby steps."
Gates told reporters after the talks that Washington was concerned that the longer the instability dragged on the more likely Iran, a Shiite theocracy, was to try to meddle in Bahrain's affairs.
On Monday, US National Security Council spokesman Tommy Vietor urged the Gulf states 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."
In a major concession to the opposition demands, Crown Prince Salman on Sunday said he supported the creation of a parliament with full powers and pledged to tackle corruption and sectarian tensions.
But he warned that "legitimate demands should not be carried out at the price of security and stability."
The opposition has refused to negotiate until the government resigns, a condition the country's rulers have deemed unacceptable.
Foreign Minister Khalid Al-Khalifa, in comments posted Monday on Twitter, accused the opposition of shifting their demands and likened the protesters to gangsters.
"All goodwill gestures were not reciprocated by (protesters)... Look where we are now," he said, adding that demonstrations amounted to "wanton, gangster-style takeover of people's lives."
The mainstream opposition says it is not trying to overthrow the royal family, but more radical Shiite elements have said they want a republic.