BOSTON (AP) – Democratic presidential candidate John Kerry on Sunday canceled an appearance at the U.S. Conference of Mayors 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 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 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.

A spokesman for the mayor said after the announcement that Menino’s aides had spoken with the Kerry campaign, and the two planned to meet later Sunday night.

“Mayor Menino has reached agreement with 73 percent of the city’s unionized work force,” Seth Gitell said Sunday evening. “He’s eager to get a deal done, but he cannot mortgage the city’s future.”

Rhonda Spears, spokeswoman for the U.S. Conference of Mayors, declined comment, saying the conference was awaiting official word of the cancellation from the Kerry campaign.

The police union, as well as unions for firefighters and other city workers, had protested at several conference events during the weekend, including a performance of the Boston Pops Sunday evening.

Before the decision was announced, Jim Barry, a spokesman for the police union, promised to picket any conference event that Menino attended.

“John Kerry’s not our target. It’s the host,” Barry said, referring to Menino.

AP-ES-06-27-04 2110EDT