By Scott Malone

BOSTON (Reuters) - Backers of Boston's embattled bid to host the 2024 Olympic Games on Monday defended the proposal against criticism that it will saddle the city with debt, saying new international guidelines will help lower costs.

The U.S. Olympic Committee in January passed up bigger cities including Los Angeles and Washington to pick Boston as the nation's bid for the 2024 Summer Games but its choice immediately came under fire by city residents and elected officials who worried taxpayers would be left to foot much of the proposed $9.5 billion cost of the event.

The cost of hosting the Games has soared in recent years, with Russia spending $51 billion on the 2014 Winter Games in Sochi.

Members of the International Olympic Committee have played up their new "Agenda 2020" approach as calling for a less expensive model that relies on existing infrastructure to keep costs down.

The IOC had seen Boston's bid, which relies on sport facilities that already exist at the city's many universities, as a model of a lower-cost bid.

"We don't want an all-expense four-week event that leaves the city potentially on the hook," Richard Davey, chief executive of Boston 2024, told a Boston City Council hearing.

A WBUR/MassInc poll of 509 registered voters last month found that a full 50 percent of Boston-area residents opposed the bid. About seven of 10 respondents said they were worried taxpayers would be left to foot the bill, though Boston Mayor Marty Walsh has repeatedly said he would not allow that to happen.

Walsh has lent his support to calls for a statewide voter referendum in 2016 that would either allow the bid to go forward or nix it. The IOC in 2017 will pick from a lineup of host cities that also include Paris, Hamburg and Rome.

Members of Boston's City Council said they remained uncertain about the bid.

"I want to be supportive of the Olympics, and I am, but I can't until we answer some of those financial questions," said City Councillor Frank Baker.

The No Boston Olympics lobbying group, which popped up last year in opposition, said in a statement it was unimpressed by the promises of a lower-cost bid.

"Agenda 2020 fails to fundamentally change the auction dynamic that leads to great outcomes for the IOC but leaves host cities awash in debt," it said.

(Reporting by Scott Malone; Editing by Lisa Lambert)