Bahrain has been gripped by the worst unrest since the 1990s since protesters took to the streets last month, inspired by uprisings that unseated entrenched rulers in Egypt and Tunisia.

Moderate opposition leaders urged hard-liners to cancel Friday’s march, warning it could spark serious clashes.

Witnesses said fighting broke out at a school in the town of Sar when some students launched anti-government protests on Thursday.

Protesters say they are excluded from jobs in the security forces and view Bahrain’s practice of settling foreigners serving in police will change the demographic balance, an accusation the government denies.

“During the break we went on a peaceful protest, we gathered, a few girls. Next thing we know a group of naturalized people were let into school and the school door was locked, they had iron and wooden sticks and knives,” said one student.

It was not clear whether there were any injuries but one witness said he saw an ambulance driving away one girl.

Seven people have been killed in clashes with security forces and thousands of the Feb. 14 youth movement still occupy Pearl roundabout, but the opposition is increasingly split.

Moderates led by the largest party Wefaq are calling for constitutional reforms and have called a less provocative rally on Friday that is expected to draw tens of thousands.

The coalition of much smaller parties behind the march on the royal court is calling for sweeping reforms.

No more than a few hundred are expected to join the march, but politicians and activists on all sides expect civilians to come out to block their advance.

“Tomorrow will be a tough day,” said a political source, who declined to give his name. “If they make the journey they will be met by plainclothes people and not security forces... These protesters are trying to derail the political process because in an election they could not win a seat if they tried.”

Protests in Bahrain have been peaceful since the initial clashes, but there have been repeated incidents of fighting between residents since.

Bahrain saw the first clashes between residents last week when at least a hundred residents fought with clubs in Hamad Town.