By Allison Lampert

MONTREAL (Reuters) - Canada's National Energy Board has indefinitely postponed hearings in Montreal on TransCanada Corp's <TRP.TO> proposed Energy East pipeline after protesters on Monday disrupted the first day's session, an agency spokeswoman said.

Montreal police said three protesters were arrested on obstruction charges, with two of the three also charged with assaulting a police officer.

Footage posted on Twitter by local media showed protesters at the downtown venue standing, clapping and chanting at the panel.

"Our first priority at any hearing is always the safety of all participants," NEB spokeswoman Sarah Kiley said in an email. "Once we have determined how we will hear from those intervenors in Montreal who were scheduled to present today and tomorrow, we will share that information."

Environmental groups opposed to Canadian oil sands development have fought the 1.1 million-barrel-per-day Energy East pipeline, which would carry crude oil from Alberta to Canada's Atlantic coast.

Opposition has been particularly strong in the mostly French-speaking province of Quebec, which the pipeline would need to cross on its way to the coast. Opponents include Montreal Mayor Denis Coderre, who has cited concerns the route could endanger forest and agricultural land.

Coderre told reporters on Monday the public needed answers on the number of jobs that would be created from the pipeline and TransCanada's contingency plan in case of a spill.

Unionized workers hoping to benefit from the construction work estimate the pipeline would create 2,000 jobs over three years in Quebec, where private investment in large projects has been hit by weak commodity prices.

"Until 2014, there was a shortage of workers," said construction union representative Eric Verdon, who gathered with unemployed members to protest in favor of Energy East. "Now they can't find jobs."

Calgary-based TransCanada also proposed building the Keystone XL pipeline, which was denied a U.S. presidential permit by Barack Obama last year, and is a frequent target of environmental protesters.

"We are standing by and ready to respectfully and constructively begin the sessions in Montreal after five such productive sessions in New Brunswick – and we will be ready when the sessions resume," TransCanada said in a statement.

Energy East has had several setbacks in Quebec. In March, the provincial government filed an injunction against the pipeline to force an environmental review, which TransCanada later agreed to.

(Additional reporting by Nia Williams in Calgary, Alberta; Editing by Bill Trott and Peter Cooney)