The annual Gay rally highlights the deep divide between the city's secular and religious communities, with marchers demanding to exercise their civil rights and opponents claiming the march debases the Holy City. Opposition to the march has generated violence in the past.
Even before the parade began Thursday, police arrested a 32-year-old ultra-Orthodox man who was carrying a homemade explosive device, according to police spokesman Mickey Rosenfeld. Under questioning, the man said he had planned to plant the explosive along the parade route, Rosenfeld said.
Ultra-Orthodox Jews have rioted repeatedly over the past week, burning tires, assaulting policemen and damaging police cars. Rosenfeld said 22 policemen were injured and 110 people were arrested.
Sheik Mohammed Hussein, mufti of Jerusalem and Palestine, denounced the march and criticized the government for permitting it.
"Such a march contradicts all religions and morals and the natural human way of being, " he said.
Opponents appealed to the Israeli Supreme Court to ban the march, but the justices ruled Wednesday night that it could go ahead.
At the 2005 march, an ultra-Orthodox man stabbed three marchers. Last year, the street parade was canceled because of safety concerns, and gays celebrated at a sports stadium on the edge of the city.
On Thursday, 7,000 police will guard the 5,000 expected marchers, Rosenfeld said. The parade route has been limited to 500 meters (yards), past the landmark King David hotel to a nearby park, where a rally will be held.
Many main streets in the downtown area were closed hours before the march, and public transportation has been routed away from the city center. A fleet of ambulances was standing by in anticipation of possible violence.
Israel's Magen David Adom rescue service said in a statement it was preparing an "unprecedented operation," readying 200 medics, 45 ambulances, 11 mobile intensive care units and a field command center. Additional medics and ambulances will be on standby, the statement said.
Jerusalem's Gay Pride parade has in the past been a relatively modest affair, with none of the flamboyant costumes or nudity common at similar events elsewhere in the world, or even in the nearby Israeli city of Tel Aviv.
The annual Gay Pride march in Tel Aviv usually proceeds without incident.