Manama (Reuters) - Bahraini police fired tear gas and sound grenades to break up protests by anti-government demonstrators on Saturday as they tried to march towards Manama's Pearl Roundabout, centre of a failed uprising last year.

A Reuters reporter said police also detained two American activists who came to show solidarity on the eve of the first anniversary of protests led mainly by the Shi'ite majority for democratic reforms in the Gulf Arab state.

An official said they would be deported for giving false information about their trips on entering the country.

Bahraini forces crushed the movement, with help from Saudi Arabia and other Gulf states, but escaped heavy censure from the United States, which shares Saudi fears that empowering Shi'ites in Bahrain would expand Shi'ite Iran's influence in the Gulf.

The U.S. Fifth Fleet is based in the island kingdom.

Groups of several hundred activists gathered at different points around Manama's old market district in an apparent effort to evade riot police, before marching towards the roundabout, now renamed al-Farouq Junction.

"To the roundabout, to the roundabout," chanted protesters, led by prominent rights activist Nabil Rajab. Behind them, police using megaphones warned the crowd that the march was unauthorized and they should disperse. Police then fired tear gas and sound grenades at the march.

Riot police seized the two foreigners, Huwaida Arraf and Radhika Sainath, part of a team of activists calling themselves Witness Bahrain who are monitoring protests this week.

Police fired sound bombs to break up a crowd of women protesters in an altercation with police over the arrest of the women activists.

Rajab said the protests would continue. "This proves to everybody that peoples' spirit is still alive and coming back, and we're not going to go away," he told Reuters.

Demonstrations have grown in number and frequency as the February 14 anniversary of the uprising approaches. Youth protesters in Shi'ite villages have also clashed with security forces, throwing petrol bombs at police lines and blocking roads with burning tires.

Activists say at least two people have died in police custody in the past month and others have died from apparent effects of tear gas. The government disputes the causes of death.

Bahrain's Sunni rulers have given parliament some more powers of scrutiny over ministers and budgets, but are resisting opposition demands that the elected parliament be given the power to approve cabinet appointments.

(Reporting by Andrew Hammond; Writing by Mahmoud Habboush; Editing by Ben Harding and Alistair Lyon)