Police in Bahrain have fired tear gas and clashed with students in a raid on a secondary school in the capital, Manama, reports say.
Officers stormed the Jabreya school for boys after students staged a protest demanding the release of a colleague arrested on Monday, activists say.
About 100 people have been arrested this month amid growing tension ahead of Sunday's F1 Grand Prix in Bahrain.
The kingdom has been rocked by anti-government protests since early 2011.
The latest incident comes a day after a car bomb blew up in Manama, though without causing injuries.
An opposition group calling itself the February 14 movement has said it was behind the blast.
Activists tweeted pictures purporting to show clouds of tear gas at Jabreya school, with people wincing and covering their faces.
Images also showed dozens of spent canisters of tear gas and stun grenades. Unconfirmed reports said there had been injuries.
One father, Mohamed Jaber, went to the school to collect his son but was told by police to leave, the Associated Press news agency reported.
The Ministry of Interior said on its twitter account "police dealt with a group of thugs outside Jabriya school according to legal regulations".
The clashes erupted when police went to break up a protest calling for the release of 17-year-old Hassan Humidan, who was arrested at the school on Monday.
Activists and protesters have called for the Grand Prix to be called off because of Bahrain's human rights record.
For the government and its supporters, holding the F1 race demonstrates to a global audience that after two years of unrest the kingdom is stable and back on track.
However an all-party parliamentary committee of British MPs at a news conference on Tuesday called for sponsors and drivers to withdraw from the race and for the F1 boss Bernie Ecclestone to cancel it.
Chairman of the All-Party Parliamentary Group for Democracy in Bahrain, Andy Slaughter, told the BBC: "There is a close relationship between the race and repression by a regime that is using F1 to try and establish normalcy."
Mr Slaughter called the race a "propaganda exercise", adding "the irony is that this week the level of repression, which goes on week in, week out, is stepped up in a bid to say that it's business as usual."
There have been almost daily clashes in the tiny Gulf island kingdom since security forces used birdshot and tear gas to quash a three-day-old peaceful protest at Manama's Pearl Roundabout on 17 February 2011.
As violence escalated 35 people, including five police officers, were killed, hundreds more were hurt and thousands jailed in February and March 2011.
Since then, opposition and human rights activists say more than 50 people have died, a figure which the government disputes.