Police: Pack of teens knock guy to ground, rob him at knifepoint

Boston Police report arresting three teenagers - 19, 15 and 14 - on armed-robbery charges following an incident on Harvard Street in Dorchester around 7 p.m. on Tuesday.

According to police, the three threw a guy to the ground:

Once on the ground, the victim says that one of the suspects showed a knife and demanded his wallet and cell phone. A search of the area enabled officers to locate the victim’s wallet.

Police say a plainclothes officer who happened to be driving by spotted the victim on the ground, one of his attackers standing over him and the other two standing there. When he got out of his car and announced himself, they ran, police say, adding he then got back in his car and followed them to Westville Street, where responding uniformed officers arrested them.