The vast crowd were demonstrating to demand a law guaranteeing their right to avoid Israel's mandatory military service -- long promised by Prime Minister Benjamin Netanyahu.

Crowds of men set fire to pieces of tarpaulin as hundreds of police officers cordoned off several roads across the city, AFP correspondents reported.

Demonstrators packed onto the tops of buildings, petrol stations, signal bridges and balconies above a sea of fellow protesters, some of whom held signs declaring: "Better to go to prison than to the army."

A helicopter flew overhead as people gathered to take part in collective prayers.

Abraham, 27 -- who studies in a Jerusalem religious seminary known as a yeshiva and declined to give his full name -- said the goal was to preserve a lifestyle lived according to the Torah, the Jewish holy text.

"We don't go to the army not because we are selfish, but because we try to preserve ourselves, what the Torah tells us and the rabbis tell us," he told AFP.

"There were hostages, and we mourned their deaths, we prayed for them three times a day, and for the soldiers," he said, referring to the captives abducted during Hamas's October 2023 attack on Israel, which triggered the war in Gaza.

"We love everyone, secular people, soldiers... We don't hate anyone. We came to unify the ultra-Orthodox community, that only wants to honour God."

- Crackdown -

The mass demonstration follows a recent crackdown on ultra-Orthodox draft dodgers, with thousands of call-up notices ignored in recent months and several deserters imprisoned.

Under a ruling established at the time of Israel's creation in 1948, when the ultra-Orthodox were a very small community, men who devote themselves full-time to the study of sacred Jewish texts are given a de facto pass from army service.

This exemption has come under mounting pressure since the start of the war in Gaza, as the military struggles to fill its ranks.

Whether the exemption should be scrapped has been a long-running point of contention in Israeli society, with Netanyahu pledging that his government would pass a law enshrining the waiver.

But he has so far failed to deliver.

Responding to the call of two ultra-Orthodox parties -- one of which forms a key part of the ruling coalition -- men travelled from all over Israel to join Thursday's rally.

The police announced the mobilisation of 2,000 officers in Jerusalem.

In June 2024, the supreme court ruled that the state must draft ultra-Orthodox men, declaring their exemption had expired.

A parliamentary committee is now discussing a bill expected to end the exemptions and encourage young ultra-Orthodox men who are not studying full-time to enlist.

- Vital support for coalition -

The issue has placed Netanyahu's coalition -- one of the most right-wing in the country's history -- under severe strain.

In July, ministers from the ultra-Orthodox Shas party resigned from the cabinet over the issue, though the party has not formally left the coalition.

The other ultra-Orthodox party, United Torah Judaism, has already quit both the government and the coalition.

The Sephardic Shas, which holds 11 seats in the 120-member Knesset, has warned that unless military service exemptions are anchored in law, it will withdraw its support -- move that could topple Netanyahu's fragile coalition, now down to 60 seats.

Some ultra-Orthodox rabbis fear that conscription will make young people less religious, but others accept that those who do not study holy texts full-time can enlist.

Ultra-Orthodox Jews make up 14 percent of Israel's Jewish population, or about 1.3 million people, and roughly 66,000 men of military age currently benefit from the exemption.

According to an army report presented to parliament in September, there has been a sharp increase in the number of ultra-Orthodox Jews enlisting despite opposition from their leaders, but the numbers still remain low, at a few hundred over the past two years.