Local labor organizations figured that, at most, 50 people would want to join together to walk letters, written in favor of pending legislation, to U.S. Sen. Arlen Specter's office.

Then 300 people filed out of the United Steelworkers Building and headed up the Boulevard of the Allies. They made it about a block before they were stopped by police for conducting a march without a permit.

For about 20 minutes the marchers and the police held still, the officers trying to keep the marchers on the sidewalks at the boulevard and Market Street while they sorted out how the crowd could proceed.

Jack Shea, the head of the Allegheny County Labor Council, said it was just proof that there was a high level of emotion regarding the Employee Free Choice Act, which is legislation promoted by organized labor and designed to allow workers to organize more easily and end employers' ability to put off ratifying a contract for years.

Ultimately, the crowd was allowed to pass, with a police escort, as marchers stayed on the sidewalks and mostly obeyed the traffic signals.

Mr. Specter's office received about 12,000 letters, many of them hand written, from workers, both union members and not, who support the act and urged Mr. Specter to support it as well.

Mr. Specter was not in his office when the marchers arrived, but a staffer said he would make sure the senator received the correspondence.

Mr. Specter has said he does not support the legislation.