One person is in custody and a Boston EMT is undergoing emergency surgery after being stabbed by a patient in the back of an ambulance, authorities say.

Police responded to New Chardon Street, next to the Edward W. Brooke Courthouse, for an EMT in distress at 4 p.m. Wednesday. Boston EMS Chief Jim Hooley said EMTs were transporting a psychiatric patient from East Boston to Massachusetts General Hospital when the female suspect became “unruly and attacked,” stabbing a female EMT with a weapon.

He said there was no prior indication the patient was dangerous.

Police wouldn’t confirm what the weapon was, but a pair of scissors was found at the scene.

Hooley said the second EMT, who was driving the ambulance, pulled over immediately and went to the back to assist his coworker. The suspect had a chemical spray, which she used on the second EMT, he added.

“These EMTs did not deserve this, every day they go out and they help people, they save lives and they don’t deserve to be attacked. It is also a reminder that this is a dangerous job they have in our first responder family,” said police Commissioner William Gross.

The suspect, only identified as a female, was taken into custody on scene after being subdued by police.

Both EMTs were taken to MGH for non-life threatening injuries. The female who was stabbed is a 14-year veteran with the department, officials said.

“Our first responder family faces danger every day, there is no such thing as a routine call,” said Gross.

Police tape cordoned off New Chardon Street from Cambridge to Congress as police collected evidence around the ambulance — which still had its back doors open and lights flashing as police placed markers on the ground.

“I’m not surprised, anything can happen these days,” said Rodrigo Madrigal, who works in the area. “But there are a lot of different people who hang around here, so it’s not surprising.”

The suspect will be arraigned in the morning at Boston Municipal Court.