Montreal Mayor Denis Coderre accused Environment Canada of playing politics after it asked the city to suspend its plan to dump eight billion litres of untreated wastewater into the St. Lawrence.

At a press conference on Tuesday, Coderre didn’t pull any punches when asked about the move from the environmental authority earlier in the day.

"From Day 1, I've been doing my homework. There's some people that have been playing politics behind the backs of Montrealers," said Coderre.

Environment Minister Leona Aglukkaq issued a statement on Tuesday saying that she wants more time to assess the environmental impact of the proposal.

"The St. Lawrence River is one of Canada's most important waterways, acting as a home to several species of whales and fish while providing millions of Canadians their drinking water," Aglukkaq said. "What's more, it's also enjoyed by many Quebecers for recreational purposes.

"The proposed plans by the City of Montreal to dump billions of litres of untreated raw sewage into this important river is very concerning -- as many citizens have noted publicly... I ask that (Montreal) Mayor (Denis) Coderre halt his plans while a proper assessment is done."

But Coderre said that he contacted the ministry just last week asking if it had any further queries about the plan, which he said has been in its possession since September 2014.

"We were asking … Environment Canada … do you have any other questions? Do you have any other documents that you need?" said Coderre.

"Well, if they don't answer it is because they have everything in their hands … and the rest is politics."

The massive sewage dump is part of work to move a snow chute in a broader plan to demolish the city's downtown Bonaventure Expressway.

The weeklong dump is scheduled to begin on October 18.

City officials have backed the plan because they say construction will temporarily shut down a nearby treatment facility and there is no other place for the sewage to go.

Coderre insisted that experts say "there are no other options," even if he knows the "image is quite bad."

"We've been asking all the questions … and we've been doing our homework," he said.

Coderre said Environment Canada can "block the project," but he invited representatives to sit down in a café in Montreal to discuss it further.

"If they want to be serious about it, they don't want to play politics … they want to be responsible and accountable, sit with us and we'll find a solution," he said.

Coderre pointed out that the city also approved similar sewage dumps in 2003 and 2005, both of which received approval from Environment Canada.

Aglukkaq said earlier on Tuesday, that she has asked her department if they are ways to throttle the plan.

When asked by a reporter how the sewage dump into the nearly 1,200 kilometre river was safe for the environment, Coderre said "because the experts say so."

He added that oxygen in river would help dissolve the waste.

Quebec's environment minister, David Heurtel, agrees with Coderre's assessment, and said earlier this week that while the plan isn't ideal, the dump needs to go forward.

However, the plan is facing growing opposition on both sides of the St. Lawrence.

An online petition created by Montreal resident Xavier Nonnenmacher called "The St. Lawrence is not a garbage" has garnered 70,000 signatures.

Jici Lauzon, the Green Party candidate for the Quebec riding of Pierre-Boucher-Les Patriotes-Vercheres, says he is working with lawyers and plans to file an injunction.

New York State Sen. Patty Richie also wrote a letter outlining her concerns to the International Joint Commission -- a bilateral group tasked with protecting the St. Lawrence.

She believes the dump could set a dangerous precedent, and the idea of dumping "the equivalent of 2,600 Olympic-sized swimming pools filled with wastewater" requires further study.

With files from The Canadian Press