A major probe into Quebec's construction industry has begun piling up a series of arrests, as authorities nabbed two more people Tuesday for alleged municipal corruption.

After a quiet start, the high-profile police probe nicknamed Operation Hammer has now yielded six arrests in recent days - including the ex-mayor of a Montreal borough.

Police described the pair detained Tuesday as businessmen involved in the construction industry. They allegedly offered money to a Quebec City bureaucrat in exchange for a permit for a housing project.

"The (municipal employee) was approached by these individuals and was offered sums of money to obtain certain favours," said provincial police spokesman Richard Gagne.

"The investigators aren't done in this case yet."

Gagne said the municipal employee wound up co-operating with police, and the money was seized.

Quebec has been rocked by allegations of criminal collusion in a scandal that involves the construction industry, politicians and the Mafia.

Facing a chorus of calls for a public inquiry last fall, Premier Jean Charest instead launched the multimillion-dollar Operation Hammer, which comprises Crown prosecutors and several police forces.

The men arrested Tuesday included a 52-year-old Montreal man detained at a construction site in Quebec City and another man, aged 60, nabbed at his home northwest of Montreal.

Neither man has been identified pending a scheduled arraignment in a Quebec City courtroom, which is postponed until Wednesday morning.

The alleged events took place between January and April of this year.

Provincial police also executed three search warrants on Tuesday related to the case -- in two offices and at a construction site -- in Montreal, Longueuil and Quebec City.

Operation Hammer began last October, and for about six months it had not yet yielded any arrests. Police have now made a half-dozen within days as cases begin to come together.

In less than a week, the Crown has charged six people in three separate cases including a former mayor of Montreal's Outremont borough.

Police have said other arrests are forthcoming.

The accused so far include Stephane Harbour, the former mayor of upscale Outremont, and two other high-ranking local officials, Yves Mailhot and Jean-Claude Patenaude.

Those three men face a variety of charges including fraud, breach of trust, falsifying documents and conspiracy. They are to appear in court on May 6.

Harbour, a former member of Montreal's powerful executive committee, quit in disgrace in 2007 following a spending scandal and a scathing auditor's report about questionable spending practices.

In another Operation Hammer case, Mario Villeneuve, an employee with the town of Beaconsfield, also faces charges of breach of trust and municipal corruption. He is also scheduled to appear in court on May 6.