Montreal Mayor Denis Coderre was the first scheduled speaker this morning but instead walked out a few minutes after a man ran screaming towards the commissioners.
He was grabbed by security but overpowered them and was joined by several other protesters who chanted, held hands and prevented proceedings from getting going.
The NEB hearings director cancelled proceedings and said the energy regulator would try to resume Tuesday.
Many Quebec activists and politicians, including Coderre, have opposed the TransCanada (TSX:TRP) project due to transport crude oil from Alberta to New Brunswick.
The hearings are set for all week in Montreal before moving to Quebec City the week of Oct. 3.
About 150 people protested against the pipeline outside the downtown Montreal venue where the hearings were scheduled to take place.