Canada's main stock index was on course for its worst session in nearly five months on Monday, hit by a plunge in energy stocks as crude prices retreated on an OPEC+ decision to boost output.

The energy sector dropped 2.8% as US crude prices were down 3.7% a barrel, while Brent crude lost 3.4%.

At 9:37 a.m. ET (13:37 GMT), the Toronto Stock Exchange's S&P/TSX composite index was down 246.07 points, or 1.23%, at 19,739.47.

The commodity-heavy index was also weighed down by a 1% fall in the materials sector, which includes precious and base metals miners and fertilizer companies, as gold futures ticked 0.1% lower to $1,812.5 an ounce.

TSX eyes weekly loss on energy tumble

The financials sector slipped 1.5%, while the industrials sector fell 1.1%.

Lumber provider Interfor Corp fell 6.2%, the most on the TSX, and the second-biggest decliner was miner First Quantum Minerals Ltd, down 6%.

On the TSX, 29 issues were higher, while 199 issues declined for a 6.86-to-1 ratio to the downside, with 21.54 million shares traded.

The largest percentage gainers on the TSX were OceanaGold Corp, which jumped 1.7%, and Tricon Residential Inc , which rose 2.0% after the rental housing company announced a joint venture to acquire more than 18,000 single-family rental homes for about $5 billion.

TSX falls on energy declines, weak home sales data

The most heavily traded shares by volume were Royal Bank of Canada, Baytex Energy Corp, and Bombardier Inc.

The TSX posted no new 52-week highs and no new lows.

Across all Canadian issues there were 13 new 52-week highs and 22 new lows, with total volume of 49.78 million shares.