Toronto's main stock market index sank more than 3 percent to its lowest point in nearly a year on Monday as a downgrade of the U.S. credit rating shook investor confidence globally and drove commodity prices sharply lower.

The move on the resource-heavy index tracked world stocks, which tumbled on deep-rooted jitters about Standard & Poor's cut to the top-tier AAA credit rating of the United States.

"That's really what has energized this move on the downside. We've sauntered around all year basically on the weaker side, and this has only emphasized that," said Fred Ketchen, director of equity trading at ScotiaMcLeod.

"And in fact, it has put the downside a whole lot more in focus."

At 11 a.m., the Toronto Stock Exchange's S&P/TSX composite index was down 370.94 points, or 3.05 percent, to 11,791.23. All 10 main sectors were on the decline, with six of them down more than 3 percent.

The index tumbled in early dealings to 11,716.40, its lowest since August 27, 2010. The index has ended lower in seven of the last nine sessions, and has lost almost 10 percent in that time frame.

The weakness was sharpest in the oil and gas sector, which was down 5.46 percent and followed the price of oil lower. Crude dropped close to 4 percent and was under $84 a barrel as concern over economic growth spread.

Top decliners were Suncor Energy, down 5.16 percent at C$30.67, while Canadian Natural Resources was off 4.3 percent at C$33.74.

Economically-sensitive financial stocks, down 3.1 percent, were also steep decliners, with Toronto-Dominion Bank down 3.78 percent at C$71.89, and Royal Bank of Canada shedding 3.3 percent to C$48.40.

Shares of North American base metal miners tumbled, along with the price of copper, zinc, and nickel, First Quantum fell 7.8 percent to C$107, while Teck Resources declined 7.35 percent to C$38.56.

But gold-miners were seen as a relative safe-haven investment, with the price of bullion vaulting to a record over $1,715 an ounce. Goldcorp was up 3.4 percent at C$46.90, topping the minority of advancers that were mostly gold miners. Barrick Gold gained 2.45 percent to C$45.99.

All told, materials, home to gold-mining stocks, were down 1.06 percent with the index's gold subgroup helping to cushion losses, up 2.33 percent.

The 11,750 level for the main index was seen as a key level, with analysts warning a convincing move through that mark might trigger a further slide.

"We've stabilized around the 11,750 psychological barrier, so we'll be looking today to see if the market closes above that mark," said Fergal Smith, managing market strategist at Action Economics.