The largest gathering ever of world leaders this weekend promises tactical headaches, gridlock nightmares and a whopping overtime bill for the Big Apple's police department.

Hundreds of city cops were expected work around the clock with federal and foreign law enforcement officials to guard the more than 60 heads of state - kings, presidents and prime ministers - in town for the United Nation's World Summit for Children."It's a headache, no two ways about it," said Deputy Chief William O'Sullivan, commanding officer of the police department's intelligence division.

Among those expected to attend the summit are President Bush, British Prime Minister Margaret Thatcher, Czechoslovak President Vaclav Havel, Israeli Prime Minister Yitzhak Shamir, President Thurgut Ozal of Turkey, and the ousted emir of Kuwait, Sheik Jaber Al-Ahmad Al-Sabah.

O'Sullivan said authorities were especially concerned about some 20 high-risk heads of state, declining to name exactly who, saying only that a "significant number" were Arab leaders.

O'Sullivan said the crisis in the Middle East definitely led to beefed up security.

"We have prioritized people for their threat potential. There's an awful lot of factors involved," the chief said, adding, "I can't discuss specifics."

Some 50 officials from the department's traffic, intelligence and patrol units met Wednesday with authorities from the State Department, FBI and Secret Service to coordinate efforts to guard the world leaders, said Assistant Chief Michael Walsh, commanding officer of the Manhattan south patrol borough.

The police force in midtown and lower Manhattan, where most of the activities will take place, will be doubled over the weekend and nearly tripled on Monday in accordance with the goings-on at the U.N., Walsh said.

The State Department was expected only to partially reiumburse the city for the overtime cost of adding 800 extra officers on Saturday, 950 added cops on Sunday and 1,600 extra officers on Monday, Walsh said.

Walsh said he did not know how much the money the massive security detail and traffic control operation would cost the department, but a spokesman for Mayor David Dinkins said the bill was sure to run in the millions of dollars.

Some of the officers will "freeze" the streets wherever Bush and his 40-car motorcade plans to go - an action authorities said will mean delays for both pedestrians and drivers throughout the weekend.

Inspector John Murphy, commanding officer of the department's traffic division, said drivers should expect delays in lower and midtown Manhattan as well as on the streets surrounding the U.N., the Waldorf Astoria Hotel, where Bush was expected to be staying, and the Jacob Javits Convention Center.