Martin Dumont, who worked for the mayor's party as a political organizer, told the Charbonneau commission that Tremblay turned a blind eye to illegal campaign financing.
Among those demanding that the mayor explain himself is Jean-François Lisée, the Parti Québécois minister responsible for Montreal, who said the status quo is unacceptable and it is time for Tremblay to answer the allegations made before the commission.
Jacques Duchesneau, Coalition Avenir Québec's anti-corruption guru, said the mayor has lost his legitimacy and should step aside.
Following her inauguration address at the Quebec National Assembly, Quebec Premier Pauline Marois told CBC's French services, Radio-Canada, she thinks Tremblay needs to reflect on the situation.
"I will talk to Mayor Tremblay. It has not been done yet. However, what I say to Mayor Tremblay is that he needs to have a deep moment of reflection and give explanations about the revelations we've been hearing at the commission," said Marois. "His credibility is undermined, but, the decision is his. I can understand that citizens are appalled. I am too from what I hear."
Earlier on Wednesday, Tremblay failed to appear at a news conference on culture, and he postponed a speech about his economic legacy that he intended to give on Friday.
Event organizer Michel Leblanc, president of the Board of Trade of Metropolitan Montreal, said he received a phone call from Tremblay on Wednesday morning.
"The reality is that he thinks people aren't listening to his economic message. That's what he told me," Leblanc said.
Tremblay had originally organized Friday's speech as an opportunity to lay out his economic plan for the city, timing the address to coincide with this week's tabling of the city budget.
Today, Quebec's biggest public sector union added its voice to the growing calls for Tremblay's resignation.
"Gérald Tremblay no longer has the credibility and the confidence necessary to lead the Montreal mayor's office," said Gaétan Châteaneuf, head of the CSN's Central Montreal Council.
If the mayor does decide to quit, the timing will be important. If he were to leave before Nov. 3, one year ahead of the next municipal election, an early vote for the mayor's office will be held.
If he quits after that date, according to provincial law, he can be replaced by city council without an election.
Union Montréal councillor quits party
A city councillor elected under Tremblay's Union Montréal banner has also turned his back on his party in the wake of the corruption allegations.
Frantz Benjamin is the first member of the party to quit since allegations of kickbacks and financing irregularities started surfacing several weeks ago in testimony given at the commission.
Tremblay has denounced the allegations against him. He is urging citizens to reserve judgment until the Charbonneau commission concludes.