The proposed 12.5-kilometre light rail transit line would run from Tunney's Pasture, just west of downtown, to Blair station in the east, and includes a 3.2-kilometre transit tunnel beneath downtown Ottawa. (City of Ottawa)

The city's light rail public-transit plan was a major topic during a debate Friday featuring Ottawa's four leading mayoral candidates.

Andrew Haydon lashed out at the city's planned $2.1-billion light rail plan, while fellow candidate Larry O'Brien, Jim Watson, and Clive Doucet defended it.

The plan, funded by three levels of government, would see the construction of a 12.5-kilometre light rail line from Tunney's Pasture to Blair Station, and a 3.2-kilometre tunnel through the downtown.

Haydon called the plan a "hoax" that would make contractors rich and taxpayers poor.

He also criticized Watson for softening on the issue. The day before the plan was first approved by city council in January, Watson said he was concerned about the project's price tag, that he would rethink the downtown tunnel, and that he might consider a "BRT" — a bus-only plan.

On Friday, Watson said ditching the LRT plan would cost taxpayers money and the city its credibility, though he also said the plan should be managed by the province and an independent private-sector board, rather than the city.

Watson said it was time to move forward and that if the city reversed its decision, it would set the city back.

O'Brien, is the incumbent, has been a strong proponent of the plan.

Doucet supported a light rail plan, but one that is substantially different from the current plan, including advancing the timeline but holding off on the tunnel.

Both O'Brien and Doucet said the plan is affordable, but Haydon said he believes the cost of the plan would double during construction.

Candidates will take part in debate on the environment Sunday night.

The election is Oct. 25.