Saudi Arabian military vehicles and soldiers have crossed over into Bahrain where there are continuing protests against the ruling royal family.
Witnesses say more than 150 vehicles and up to a thousand soldiers arrived via the causeway linking the two kingdoms.
It appears the Saudis were asked to come in by the Bahraini government, and witnesses say the soldiers are heading towards the area where the royal family lives.
But opposition groups are voicing their anger at the development.
The leader of opposition group Al-Waad, Ibrahim Sherif Al Sayd, said: “We say to our brothers in the Gulf, your army is welcome when our country faces danger from outside, but we will consider them an occupation when you come to oppress the people.”
Protesters gathered up used canisters of tear gas and rubber bullets to show to reporters. They say they were used by police to quell the unrest which has been going on since last month.
Roadblocks have been set up around the financial district and around Pearl Square which has become the focus of the demonstrations.
Clashes on Sunday resulted in some of the most violent fighting since troops killed seven people last month.
The United States is urging the six countries in the Gulf Cooperation Council to show restraint and respect the rights of the people of Bahrain.