"Hundreds of demonstrators gathered in (the ultra-Orthodox neighbourhood of) Mea Sharim, stopped traffic and threw stones and bottles at police," Micky Rosenfeld told AFP, adding that five of them were arrested.
The demonstration came a month after parliament adopted a law that will compel ultra-Orthodox Jews to either serve in the military or perform civilian service.
Last year, the cabinet agreed to end a practice under which tens of thousands of ultra-Orthodox men were exempted from military service if they were in full-time study at a yeshiva (Jewish seminary).
In broader Israeli society, military service is compulsory for both sexes, with men serving three years and women two.
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The new law sets out sanctions against draft dodgers -- including imprisonment. That has enraged the ultra-Orthodox leadership, who say it would be tantamount to jailing people for practising their faith.
In March, more than 300,000 ultra-Orthodox took to the streets of Jerusalem in a mass prayer vigil to protest against the impending legislation.
The new law is seen by many Israelis as amending the historic injustice of an exemption handed to the ultra-Orthodox in 1948, when Israel was created. At that time they were a small segment of society.
And many full-time yeshiva students live off government stipends, which are provided for several years after the age of 18.