MONTREAL, Sept 2 (Reuters) - Canadian Prime Minister Justin Trudeau, struggling ahead of a Sept. 20 election, came under concerted fire at a debate on Thursday from opponents who said he had no business calling an election during the COVID-19 pandemic. Trudeau and the leaders of three other parties took part in a sometimes heated two-hour French language debate, often speaking over each other. But unlike previous encounters, none of the four appeared to land a knock-out blow. Trudeau triggered the vote last month just two years into a four-year mandate, saying Canadians needed to decide whether the Liberal government was right to spend billions of dollars to protect people and businesses from the pandemic. Trudeau"s ruling Liberals had a minority government, which left him dependent on other parties to govern. Surveys show that the Liberals" early big lead has vanished, leaving them neck and neck with the Conservatives of Erin O"Toole. "We can work to do big things during a pandemic but we need a clear mandate ... to understand what Canadians want for the next years," Trudeau said. "This is not the time for an election, during a pandemic," O"Toole retorted. Yves-Francois Blanchet, leader of the separatist Bloc Quebecois, then told Trudeau that the Parliament could easily have lasted four years. A few minutes later Trudeau said that if he only obtained another minority there could be another election in 18 months" time, which would make it the eighth federal vote in 19 years. Jagmeet Singh, leader of the left-leaning New Democrats, said after the debate that Trudeau"s remark showed he did not want to work with other parties. The Liberals often relied on the New Democrats for support in the last two years. The debate - the first of three - took part in Montreal, the largest city in the influential and predominantly French-speaking province of Quebec. Liberal insiders say public unhappiness with the election, which is taking place two years early, is partly responsible for the party"s slow start. They also cite voter fatigue with Trudeau, who took power in late 2015 and retained office with a minority in 2019. Quebec accounts for 78 of the 338 seats in the House of Commons and is crucial for any party seeking office. The four leaders spent considerable time discussing Quebec matters, including a 2019 law on secularism which bans public employees from wearing religious symbols such as hijabs and turbans on the job. Polls show the law - currently being challenged in the Quebec courts - is popular. But critics denounce it as racist and Trudeau said he would if necessary be prepared to challenge it federally, which could cost him political support. O"Toole made clear that as prime minister he would not act. The debates can be decisive. During the 2019 TVA debate, Trudeau and other leaders accused then-Conservative leader Andrew Scheer of plotting curbs on abortion. Scheer became flustered and his campaign never recovered. Next week the Canadian Broadcasting Corp will host an English and a French debate.