SEPTA's largest union went on strike early this morning - crippling a transit system that averages more than 928,000 trips every weekday in the city.

The strike came after hours of negotiation yesterday that ended when, according to Willie Brown, president of Transport Workers Union Local 234, both sides agreed that they had gone as far as they could go.

The strike, Brown said early this morning, would begin at 3 a.m., threatening to cripple this morning's rush hour.

Gov. Rendell, who along with Mayor Nutter addressed reporters in the lobby of the Park Hyatt at the Bellevue, where negotiations had been under way since 10 a.m. yesterday, called the decision to strike before dawn "irresponsible."

Rendell said the union action would cause chaos with morning transit riders who would not know what is happening until after they wake up.

"This is an outrageous action," Nutter said.

All bus, subway, and trolley service in the city, as well as the Frontier Division buses in Bucks, Montgomery, and Chester Counties, would be stilled by a strike.

Regional Rail service would not be interrupted because those crews are covered by separate contracts. Also continuing to run during a strike would be Victory Division buses in Delaware County, most operating out of the 69th Street Terminal in Upper Darby; Route 101 and 102 trolley lines and the Norristown High Speed Line; "LUCY" buses in West Philadelphia between 30th Street Station and University City destinations; and Paratransit service.

Among the thousands of people in the region who rely on SEPTA to get around are Philadelphia School District students who attend school outside their neighborhoods. Last week, the school district said that classes would not be canceled. It was unclear what arrangements would be made.

In the end, it was a difference over wages that sparked the walkout. Earlier yesterday, transit officials disclosed that both sides had reached a tentative agreement on health care and were reportedly close on wages.

"Nobody wants to leave something on the table," U.S. Rep. Bob Brady, who had been involved in the negotiations since last week, said during yesterday evening's break.

But Brown, in a telephone interview, painted a different picture early today.

"They wouldn't provide the proper numbers" during negotiations, Brown said. "When it comes right down to it, they've underfunded our pension for years."

The talks broke down shortly before midnight.

Rendell said the union chose to walk away from an "excellent" contract offer that includes 11 percent in wage increases over five years, and 11 percent increase in pension contributions, and no increases in workers' contribution for health care.

"Think about that," Rendell said. "Whose pension has been increased in this day and age?"

According to TWU officials, SEPTA management has proposed no wage increase for the first two years of a four-year contract and a 2 percent increase in each of the final two years. It also wanted to increase worker contributions to health coverage from 1 percent to 4 percent and freeze the level of pension benefits.

The union wants a 4 percent raise each year and health contributions to remain 1 percent. It is also seeking an increase in pension contributions from $75 to $100 for every year of service.

When a strike was threatened last week - with the World Series moving to South Philly - Nutter had called a walkout "pretty unimaginable," and Brady increased his efforts to broker an agreement.

Rendell urged SEPTA's 5,100 unionized bus drivers, subway and trolley operators, and mechanics not to strike during the World Series, saying it would give Philadelphia "a little bit of a black eye" on the national stage.