OTTAWA, Sept 21 (Reuters) - Erin O"Toole, the leader of Canada"s main opposition Conservative Party, conceded defeat on Tuesday after failing to block Prime Minister Justin Trudeau"s Liberals from securing a third term. Speaking to supporters in his home electoral district outside Toronto, O"Toole said he had called Trudeau to congratulate him. TV networks predicted the Liberals won a minority government. As of 12:40 a.m. (0440 GMT), the Conservatives were leading or elected in 121 of 338 seats, while the Liberals were on 157.