JERUSALEM -- Thousands of ultra-Orthodox Jews blocked highways across Israel on Thursday and clashed with security forces to protest government plans to draft them into the military, police said.

The simultaneous rallies in numerous locations caused large traffic jams, said police spokesman Micky Rosenfeld.

In Jerusalem, some 400 activists tried to block the entrance to the city but were dispersed with water cannons and 15 were arrested. In the southern city of Ashdod, the demonstrators hurled stones at the police, injuring two policemen. Rosenfeld said eight protesters were arrested there.

Elsewhere, a major highway in central Israel was blocked by about 2,000 protesters. Police on horses beat back demonstrators with clubs and used to stun grenades to clear the roads.

The riots follow a Supreme Court ruling this week ordering funding halted to ultra-Orthodox seminaries whose students dodge the draft. Finance Minister Yair Lapid's freezing of the funds sparked angry warnings from ultra-Orthodox leaders.

Ultra-Orthodox Jews have for years been exempt from military service, which is compulsory for Jewish Israelis. The arrangement has caused widespread resentment among Israel's secular majority and featured prominently in last year's election.

A new system that would gradually reduce the number of exemptions and require all to register for service awaits parliamentary approval.

The ultra-Orthodox have been demonstrating against the plan and condemned the court decision. They claim the military will expose their youth to secularism and undermine their devout lifestyle.

The issue of army service is at the core of a cultural war over the place of ultra-Orthodox Jews in Israeli society. The ultra-Orthodox, who make up about 8 per cent of Israel's 8 million citizens, have largely been allowed to skip compulsory military service to pursue their religious studies. Older men often avoid the workforce and collect welfare stipends while continuing to study full time.