Critics slammed boosters of a Boston Olympics for a lack of transparency at the first public debate on the city's bid yesterday.

Last week, a private group submitted a formal proposal for Boston to host the 2024 Summer Games. The details have not been made public. At the debate, Chris Dempsey of the group No Boston Olympics says the whole process has been shrouded in secrecy.

"Boston 2024 has been around for more than 1 year," Dempsey said. "And in more than one year it's held zero public meetings to have opportunity to get input from the public."

Juliette Kayyem of Boston 2024, the group backing the bid, says the debate in Boston has been bigger than in any of the other U.S. cities under consideration

"The idea that a group of people in a secret room is deciding the fate of Boston is just not true," she said.

Kayyem says the proposal has been kept private because Boston is competing with Los Angeles, San Francisco and Washington to be the U.S. nominee to host the games.

Kayyem says in putting together its bid, the group has learned from the mistakes of other host cities.

"Where the U.S. Olympic Committee is and where the International Olympic Committee are is focusing on an Olympics that’s sustainable, that is cost effective and that is durable for the future," she said. "That’s where Boston is now."

Kayyem says the event will cost $4.5 billion in private money, plus another $5 billion in public money for roads and infrastructure — which she says Boston needs anyway. That’s only a fraction of the $51 billion price tag for the Sochi games.

Dempsey predicts billions in cost overruns.

“And what would happen is that we would need to call on public resources to pay for those overruns," he said.

The U.S. Olympic Committee will decide whether to nominate Boston, San Francisco, Washington or L.A. in mid-January.