(Adds Trudeau rally, White House comments)
By Allison Lampert and Randall Palmer
MONTREAL/OTTAWA, Oct 20 (Reuters) - Canada's Prime Minister-designate Justin Trudeau, having trounced his Conservative rivals, will face immediate pressure to deliver on a swathe of election promises, from tackling climate change to legalizing marijuana.
Trudeau toppled Stephen Harper's Conservatives to win a majority on Monday, giving him the freedom to start implementing his campaign pledges largely unimpeded.
The 43-year-old son of former prime minister Pierre Trudeau was swept to victory with 39.5 percent of the popular vote in an election that saw the highest voter turnout since 1993.
The win marked a turn in political fortunes that smashed the record for the number of seats gained from one election to the next. The center-left Liberals had been a distant third-place party before the vote.
Trudeau campaigned on a promise of change, striking a chord with Canadians weary of nine years of Conservative rule. Harper resigned as party leader after the defeat.
"When the time for change strikes, it's lethal," former Conservative Prime Minister Brian Mulroney said in a television interview.
Trudeau, a telegenic father of three, also returns a touch of glamour, youth and charisma to Ottawa.
Britain's Daily Mirror newspaper asked "Is Justin Trudeau the sexiest politician in the world?" while an Australian news website was more direct: "The votes are in and Canada has come out of its election with a super hot new leader."
Trudeau, who kicked off his first morning as prime minister-designate by greeting astonished voters at a Montreal subway station in his home constituency, held a rally in Ottawa on Tuesday.
"This afternoon we can celebrate but the work is only beginning," Trudeau, speaking in French, told several hundred jubilant supporters in Ottawa.
The Liberal leader will have to quickly start delivering on his promises to change policy, beginning with a U.N. climate change summit in Paris in December. Major allies have told Canada it needs to commit to more ambitious targets for cutting emissions of greenhouse gases.
"Canada has also made a substantial and important commitment in advance of the Paris climate talks. We believe that it's possible that there is more that Canada can do in this regard," White House spokesman Josh Earnest told reporters at a briefing on Tuesday.
The White House also said it hopes the new Canadian government will continue to support the efforts of the U.S.-led coalition to fight Islamic State militants in Syria and Iraq.
Trudeau has pledged to withdraw Canada's CF-18 bombers from the coalition fight but maintain humanitarian aid and training.
U.S. President Barack Obama was expected to telephone Trudeau to congratulate him on his win, Earnest said.
Meanwhile, U.S. Secretary of State John Kerry said Harper's defeat will not affect Kerry's decision on whether to approve the controversial Keystone XL pipeline between the two countries.
Trudeau backs Keystone and has vowed to repair cool relations between Ottawa and Washington.
Choosing a cabinet will be one of Trudeau's top priorities before he and his ministers are sworn in over the next two to three weeks.
Former Canadian Finance Minister Ralph Goodale is among seven top contenders to run that department, a senior adviser to Trudeau said.
The Liberals plan to run a C$10 billion annual budget deficit for three years to invest in infrastructure and help stimulate Canada's anemic economic growth.
Stock investors cheered the Liberal victory, betting it would loosen government purse strings to kick-start growth.
Shares of construction firms and railways rose on the Toronto Stock Exchange along with heavyweight resource and financial stocks.
Canadian medical marijuana stocks also were higher.
The Liberals' strong showing removed the uncertainty that could have resulted from a minority government, and while the new administration plans to run deficits, it has also said it would keep corporate tax rates steady.
The Canadian dollar strengthened as fiscal policy could limit the need to cut interest rates.
"People are breathing a sigh of relief and they are looking for those areas that should show a positive impact from what the Liberals were talking about," said Irwin Michael, portfolio manager at ABC Funds.
Writing by Amran Abocar; Editing by James Dalgleish and Jeffrey Benkoe