Police are negotiating with a man with a knife who has barricaded himself and a three-year-old boy inside a community centre in Vancouver's Downtown Eastside, and officers say the stand-off could last into the night.

The man is shut up inside basement of the Ray-Cam Community Centre in the 900-block of East Hastings Street and witnesses say he is armed with a long, military-style knife.

Const. Lindsey Houghton told reporters that the suspect had an argument with community centre staff shortly before snatching the boy, who was sitting with his mother.

Houghton added that negotiations could last all night.

A man at the scene told CTV News that the boy is the son of his brother-in-law.

"He's a very good kid," the uncle said.

"Apparently, the mother was walking with the kid and he grabbed him. He grabbed him right from the mom. I don't know why he grabbed him."

He said the tot attends daycare at the community centre.

He added that the child's mother and father were both inside the community centre, where they were communicating with their son.

The suspect is a stranger and not related to the little boy. A witness told reporters the child's mother was crying while he was taken from her.

The witness said that the suspect was shouting very loudly when he took the boy.

The stand-off began around 6 p.m. and Emergency Response Team members are now on scene. Police have sent in a robot in an attempt to defuse the situation.