About 70 agents from the provincial Unité Permanente Anticorruption, or UPAC, descended on city hall, Mayor Gilles Vaillancourt's house and buildings housing the municipality's engineering and human resources departments.
It was UPAC's highest profile raid against a politician since it was created last year.
The raids began at about 3:45 p.m. ET. Staff at city hall had to evacuate the building while officers looked for documents and seized computer hardware.
"We don't have a lot of details for the moment. All we know is that there's a search. We don't know how long they'll be here. All I know is that they're conducting a search at city hall," city spokesperson Johanne Bournival said.
"I've heard that the UPAC team is at his house but he wasn't home. He had an activity linked to his mayoral duties this afternoon."
Vaillancourt has been the subject of allegations of political impropriety for at least two years. In November 2010, CBC's French-language news service reported that he offered $10,000 in illegal cash contributions to the election campaign of a Parti Québécois candidate for Quebec's national assembly, and an unspecified amount of cash to a Liberal candidate. Vaillancourt denied the accusations. Radio-Canada reporter Alain Gravel said investigators began looking into Vaillancourt around that time.
Then this summer, allegations of another $10,000 cash contribution surfaced.
Mayor for 23 years
A spokeswoman for the anti-corruption unit wouldn't say what was being seized, but she confirmed material was gathered as part of an ongoing investigation.
"There is an investigation underway and the seizures are taking place to help along that investigation," Anne-Frederick Laurence said.
Vaillancourt is not accused of any crime.
Vaillancourt has enjoyed a 23-year reign at the helm of Laval, the third-biggest municipality in the province. He was a city councillor before that, starting in 1973.
A series of Quebec construction executives, government officials and politicians have been charged with criminal offences as part of Quebec's years-long anti-corruption efforts. The mayor of the Montreal-area municipality of Mascouche and two formermayors of nearby Boisbriand have all been charged, as has the onetime right-hand man of Montreal Mayor Gérald Tremblay, former city councillor Frank Zampino.
A public inquiry, known as the Charbonneau commission, is also looking into corruption in the awarding of construction contracts and ties to politics and organized crime. A key witness testifying at the commission over the last week admitted he was part of a group of companies that colluded to rig the price of bids on City of Montreal contracts.