Updated

Democratic presidential candidate John Kerry (search) on Sunday canceled an appearance at the U.S. Conference of Mayors (search) rather than cross a promised police union picket line at the event.

"I don't cross picket lines. I never have," Kerry said as he left Mass Sunday night at Our Lady of Good Voyage chapel in South Boston.

His decision came hours after Boston Mayor Thomas Menino (search) called on Kerry to attend, calling the conference "an important event for urban America," and saying the pickets set up by the Boston Police Patrolmen's Association (search) and other union members did not constitute a legitimate picket line.

Menino has been locked in a battle with several city unions over unsettled contracts.

"We know that people on both sides have been working in good faith to resolve the situation, and we hope that they will redouble their efforts to find a resolution," said Kerry campaign spokesman David Wade.

Menino said that he spoke with Kerry on Sunday evening, and that he was "very, very disappointed" with Kerry's decision.

"I would think that he would come and talk to the mayors who are making a difference in America everyday, who are on the front lines of the issues that face working people," Menino said.

"We're very proud of the senator and his stand," said Jim Barry, a spokesman for the police union.

Detroit Mayor Kwame M. Kilpatrick said he was disappointed and angry.

"This was the opportunity for Sen. John Kerry to give a message for how he's going to help mayors and cities," Kilpatrick said. "I'm concerned whether he's going to support the mayors of this country in delivering for their citizens."

Conference President James A. Garner, mayor of Hempstead, N.Y., said in a statement released early Monday that the mayors are disappointed, and hoped to meet with Kerry soon to discuss issues facing American cities.

More than 200 mayors are attending the conference, a nonpartisan organization that brings together mayors from cities with populations of more than 30,000.