Bahraini security forces fire tear gas and stun grenades as protesters mark the first anniversary of an uprising that was crushed by military force.

Bahraini security forces fire tear gas and stun grenades as protesters gather to mark the anniversary of an uprising that was crushed by military force (Reuters)

Thousands of riot police and other members of the security forces have taken up positions around Pearl Square in the capital Manama, the centre of the violence a year ago, when some members of Bahrain's Shia majority rose up against the ruling Sunni regime.

On Monday, oppostion supporters threw firebombs and rocks at the security forces while trying to occupy the square.

The authorities have warned that they will adopt a zero-tolerance approach to anti-government demonstrations on the island base of the US Navy's 5th Fleet.

Daily battles

The government imposed martial law in March 2011 after crushing the demonstrations. Emergency rule came to an end in June, but there are almost daily street battles between the security forces and mainly Shia protesters.

Protesters reject accusations that they began the latest round of violence. One anonymous demonstrator said: "Today we sat outside our homes as a peaceful method of protest. Then the repression by these Khalifa forces began, so we have to confront them. It was before our houses. They are the ones who came in their cars."

Last year's protests coincided with uprisings in Tunisia and Egypt. By the time time martial law ended, 35 people, including protesters, police, Shia detainees and foreigners, had died. Since then, another 24 people are thought to have been killed.

Iran accused

The ruling Al Khalifa family accuses Iran of fomenting violence. Tehran denies this, and Bahrain's Shia groups reject claims that they are receiving support from abroad.

When they shout 'down with the king and up with Khomeini', that's a problem for national unity. King Hamad of Bahrain

In an interview with Germany's Der Spiegel magazine, King Hamad of Bahrain has accused his opponents of chanting support for Ayatollah Khomeini, leader of Iran's 1979 revolution.

"It's just a case of manners. But when they shout 'down with the king and up with Khomeini', that's a problem for national unity," the magazine quoted Hamad as saying.

Leading activist Nabeel Rajab led several hundred people in an attempted march to the square on Saturday, which ended with the arrest of two American activists, who were deported on Sunday.

"This is a continuous protest," he said. "There will not be one central protest with thousands of people, it will be all over."

Mainly Shia opposition parties want Bahrain's elected parliament to be given the power to form governments. The government says its reforms include giving parliament more powers of scrutiny over ministers and budgets.