BOSTON — A Boston man allegedly tried to drag a three-year-old boy off a Green Line trolley by his leg Tuesday afternoon — and threatened the terrified mother with a "finger gun" when she tried to stop him, Suffolk County prosecutors said.

The horror began around 12:53 p.m. on May 19 on an empty Green Line E trolley out of North Station.

A mother boarded with her three-year-old son. Moments later, Dualle boarded and sat one seat away from her.

He allegedly tried to "flirt." When she ignored him, he became aggressive — asking for her number, telling her "we should get out of here," prosecutors said. Another female passenger stepped in to distract her.

Between Haymarket and Government Center stations, Dualle's attention shifted to the three-year-old. He allegedly tried to tickle the boy's feet with his fingers. The mother told him to leave her son alone.

That's when, prosecutors say, Dualle made the "finger gun" — and pointed it at her.

"I'll [expletive] kill you; you understand me? I'll shoot you," he allegedly told her.

Then he turned back to the boy.

"Look he wants to come with me," Dualle allegedly said. "Look at the way he's looking at me. He wants me."

Dualle then allegedly tried to remove the toddler from his stroller — grabbing the boy's leg and pulling him up.

Another passenger and an MBTA employee intervened, pushing Dualle away. When the trolley reached Park Street, the MBTA employee forced him off the train.

Transit Police were dispatched to the westbound platform at Park Street. After a search of the area, officers spotted a man matching Dualle's description at the intersection of Chauncy and Summer streets. In a show-up identification, the mother positively identified him.

"No user of public transportation should have to experience an episode like this, particularly a parent with a young child," Suffolk District Attorney Kevin Hayden said in a statement. "I'm grateful for the passenger and the MBTA employee who helped this mother and child and I'm thankful police were able to identify and locate this defendant in fairly quick order."

Dualle is presumed innocent until proven guilty in a court of law.