At 9:52 a.m. ET (1352 GMT), the Toronto Stock Exchange's S&P/TSX Composite Index fell 87.97 points, or 0.57 percent, to 15,279.32.
The financial and energy sectors that together make up for more than half the weight of TSX slipped about 35 points each.
Enbridge and TransCanada Corp fell more than 1 percent each and were the biggest drag on the energy sector as crude prices slipped.
Royal Bank of Canada and Toronto Dominion Bank slipped more than 0.5 percent each and were the biggest lags on the heavyweight financial sector.
China raised tariffs by up to 25 percent on US products, in response to US duties on aluminum and steel imports, rekindling trade war concerns.
Oil fell to around $69 a barrel on Monday, reversing an earlier rally, as a rise in Russian production and concern about a US-China trade spat offset a drop in US drilling activity.
The materials sector, which takes up nearly 12 percent of the index's weight, along with utilities sector were the only two among the 11 sectors in positive territory.
The largest percentage gainer on the TSX was the shares of the printing company Transcontinental, which rose 6.1 percent on a deal to buy the US unit of plastics packager Coveris Holdings S.A for C$1.70 billion.
The largest decliner was Prometic Life Sciences, down 6.8 percent, after two brokerages trimmed their price targets on the stock.
Among the most active Canadian stocks by volume were Prometic Life Sciences, Aurora Cannabis, and Neovasc Inc.
The TSX posted four new 52-week highs and two new lows. Across all Canadian issues there were 11 new 52-week highs and 12 new lows.
Volume on the TSX index was 19.41 million shares, while the total volume on Monday was 31.29 million shares.