NEW YORK (AFP) - New York's school bus system ground to a halt Wednesday in the first strike by drivers in three decades, leaving 152,000 children looking for alternative ways to get to class.

The union, ATU Local 1181, did not say how long the strike, called over a disagreement with the city on job security, would continue.

Union president Michael Cordiello said on NY1 television that the strike, which also included staff who help children get on and off buses, could go on indefinitely unless Mayor Michael Bloomberg comes to terms.

"The fact of the matter is that in June, 2,500 of our members will be out of work. He's given us no choice but to fight for our jobs," Cordiello said.

"The mayor can end this strike at any time," the union said in a message urging parents to petition City Hall in support of the drivers.

Bloomberg said on Fox5 News that he would not "cave in." The dispute centers on the city's decision to open new contracts for 1,100 bus routes set to expire this year to outside bidders.

Bloomberg says competition is essential to reduce costs of $1.1 billion a year, which he said came to $6,900 per child, by far the highest rate in the United States.

The union said the plan could see senior drivers who've had their jobs for years ejected if new companies win the contracts.

"The union's hand has been forced by Mayor Bloomberg, who requested bids for contracts without critical safeguards that help ensure our children's safety," the union statement said.

"Our children deserve the best, and that is what this fight is about.

The city announced that subway tickets would be given free of charge to students and that parents forced to use cars or taxis for the school run would be reimbursed. About 1.1 million children attend New York City schools, with only a relativley small portion using the yellow buses.

"The first days will be extremely chaotic," schools Chancellor Dennis Walcott told 1010 WINS radio. "It hasn't happened in New York City in over 33 years."