A New Jersey Transit bus rear-ended a private bus carrying Canadian school students inside the Lincoln Tunnel on Wednesday, injuring more than 30 people and slowing traffic on one of the busiest routes for commuters entering and leaving New York City, authorities said.

Some commuters — one of them a pregnant woman who went into labor — found themselves stuck in the tunnel connecting New York and New Jersey for about 20 minutes after the rush-hour accident, authorities said. It happened at around 9:30 a.m. in the center tube on the New York side of the tunnel.

None of the 26 students and two adults from the Toronto school was hurt, according to Anna Caputo, spokeswoman for the Toronto District School Board. The group was heading to New York for a graduation trip, she said.

The NJ transit bus driver and 30 other people suffered bumps, bruises, neck and back injuries and other injuries that weren't considered life-threatening, city Deputy Fire Chief Stephen Lincke said. Meanwhile, a 32-year-old woman on another bus, not involved in the accident, went into labor while stuck in the tunnel and was taken to a hospital, authorities said.

Four people needed stretchers, but the rest were able to walk, said Deputy Chief Richard Gutch of the police department of the Port Authority of New York and New Jersey, which operates the tunnel. People with bloody noses were being treated at a triage station set up outside the tunnel.

Authorities said most of the injuries were on the transit bus, where rescuers had to cut the damaged door open to get people out. The bus was carrying about 60 riders from Cresskill, New Jersey, to the Port Authority Bus Terminal, the transit agency said.

Jaehoon Chung, a carpenter who was going to work on the NJ Transit bus, said the impact of the crash felt like a minor collision.

He said the passengers were stuck on the bus for 10 to 20 minutes because the door was damaged.

"We couldn't get out. The door was bent," he said. "Everyone was calm. We all waited until a cop came."

The NJ Transit but was eventually towed out of the tunnel. The Toronto bus was driven out.

The crash caused delays of up to one hour heading into New York. The two lanes of the center tube in the tunnel reopened before noon.

Pentangelo said traffic was back to normal by the afternoon rush.

The Port Authority says 42 million vehicles a year travel through the tunnel, composed of three tubes built beneath the Hudson River.