Here is an article from the Vancouver Sun. Section 801 of the Canadian criminal code allows police to arrest someone they feel might commit a crime.

Sex offender arrested at Vancouver airport
Vancouver Sun
Published: Saturday, September 15, 2007
A Prince Edward Island sex offender who is considered a high risk to reoffend was arrested at the Vancouver airport Friday, just hours after his release from prison, the CBC has reported.
Thane Moore, who has called himself a "walking time bomb," was released from Dorchester Penitentiary early Friday after serving his entire sentence in the New Brunswick prison.
He left Atlantic Canada immediately, but authorities wouldn't say where he had gone. Moore was escorted by RCMP.
Late Friday night, CBC reported on its website that Moore was immediately taken into custody after arriving at the Vancouver airport.
A spokesman for the criminal justice branch of B.C.'s Ministry of the Attorney-General told CBC that the RCMP used Section 810 of the Canadian Criminal Code, which allows officers to arrest people police strongly believe will commit a crime.
Stan Lowe, a spokesman for the criminal justice branch in B.C., could not be reached Friday night for comment.
In 1993, Moore beat and raped a young P.E.I. woman and left her for dead. Parole officials say Moore has not been rehabilitated and remains a high risk to reoffend.
Moore has signed a peace bond, saying he would not move to Whitehorse, where a number of people say they have reason to fear his going there. And he has agreed not to move to Dawson City after a public outcry when he said he would go there.
However, he is free to move anywhere else in Canada, albeit under certain conditions, among them that he report to police weekly, stay away from drugs and alcohol and not possess firearms. Moore, who is from Murray Harbour, P.E.I., must also not contact 13 people or their families, including the woman he sexually assaulted.
Because he served his full sentence, he is not under any form of supervision, nor is he required to undergo any rehabilitation.