Last season's eventual champion Sebastian Vettel races in the Bahrain Grand Prix at Sakhir [GALLO/GETTY] 

Formula One's season-opener in Bahrain next month was cast into further doubt as two GP2 races were called off at the Sakhir circuit due to bloodshed in the capital Manama.

The capital of the Gulf kingdom was rocked by sporadic clashes on Thursday, hours after riot police attacked a makeshift encampment of pro-reform protesters in the centre of the city, killing at least four and injuring dozens of others.

Thousands of overwhelmingly Shi'ite protesters have taken to Bahrain's streets this week demanding more say in the Arab state, where the Sunni Muslim Khalifa family rules over a majority Shi'ite population.

"Following the current events in Bahrain, at the request of the Bahrain Motorsport Federation, it has been decided that the remainder of the meetings which was supposed to take place at the Bahrain International Circuit is called off," organisers of the GP2 Asia series races said in a statement.

Two people died in demonstrations and clashes in Manama earlier in the week.

GP2 Asia is a regional single-seater championship similar to the main GP2 series that serves as a feeder to Formula One.

The first race of the Bahrain event was scheduled for Friday with another on Saturday.

The opening race of the F1 season is scheduled for Bahrain on March 13, with testing due there in the previous week.

Earlier organisers cancelled Thursday's scheduled practice for safety reasons.

"Due to the current local events, the medical staff normally stationed at the circuit has been temporarily called to the city's hospitals in case of an emergency," they said.

Clashes 'spread out'

An Al Jazeera correspondent, who cannot be named for security  reasons, said on Thursday that "clashes were no longer limited to one place...they are now spread out in different parts of the city".

He said that the hospitals are full of injured people after last night's police raid on the protesters.

"Some of them are severely injured with gunshots. Patients include doctors and emergency personnel who were overrun by the police while trying to attend to the wounded."

Another Al Jazeera online producer said that booms could be heard from different parts of the city, suggesting that "tear-gas is being used to disperse the protesters in several neighbourhoods".

Latest reports, however, indicated that a tense calm had descended on the capital with troops patroling the streets.

There were also reports of dozens of armoured vehicles moving towards the Pearl Roundabout, the protest site that was raided by the riot police.

Heavily-armed police stormed the traffic circle while the protesters camping overnight were asleep.

Source: Al Jazeera and agencies