CANADA STOCKS-TSX futures drop as US tariffs take effect

March 4 (Reuters) - Futures for Canada's main stock index fell on Tuesday, as U.S. President Donald Trump's new tariffs on top three trading partners of the world's biggest economy took effect, triggering a global trade war.

March futures on the S&P/TSX index SXFcv1 were down 0.5% as of 6:22 a.m. ET (1122 GMT).

Trump's 25% tariff on imports from Mexico and Canada took effect at 12:01 a.m. EST (0501 GMT), along with a doubling of duties on Chinese goods to 20%.

China responded with additional tariffs of 10%-15% on certain U.S. imports from March 10. Canada and Mexico, which have enjoyed a near tariff-free trade relationship with the U.S. for the past three decades, were poised to swiftly retaliate against their long-standing ally.

"The 25% tariff on Canada will hit exports, investment and consumption, resulting in a 2.5% to 3.0% decline in Canada's GDP," said Paul Ashworth, chief North America economist at Capital Economics.

Canadian Prime Minister Justin Trudeau said Ottawa would respond with immediate 25% tariffs on C$30 billion ($20.7 billion) worth of U.S. imports, and another C$125 billion ($86.2 billion) if Trump's tariffs were still in place in 21 days.

The Canadian dollar CAD= lost 0.3%.

The Toronto Stock Exchange's S&P/TSX composite index .GSPTSE closed down 391.88 points, or 1.54%, at 25,001.57 on Monday after the U.S. tariffs were confirmed. This marked its lowest close since January 16 and its most significant decline since December 18.

In company news, the Yomiuri newspaper reported that Seven & i 3382.T plans to reject the $47 billion takeover offer from Canada's Alimentation Couche-Tard ATD.TO and instead seek to enhance corporate value on its own.