Authorities in Bahrain have tightened security ahead of practice sessions for Sunday's Formula 1 Grand Prix.
Anti-government protesters have promised "three days of rage" to coincide with the event.
Correspondents say there is palpable unease about the fate of the race amid escalating unrest and violence.
On Wednesday night, Force India team members were forced to flee as petrol bombs were thrown near their car on their way back from the track.
The four mechanics in a 4x4 were caught in the middle of an incident as police clashed with protesters. Two members of the team have since asked to return home.
Formula 1's governing body, the FIA, only decided to go ahead with this weekend's race at the last minute.
On Friday evening, the UK's shadow home secretary Yvette Cooper called for the Grand Prix to be cancelled. She told the BBC's Question Time programme: "It shouldn't go ahead, I don't think British drivers should go. I think the Formula One should not go ahead in Bahrain."