(Adds details, updates prices)
* TSX down 63.99 points, or 0.53 percent, at 12,009.47
* Nine of the TSX's 10 main groups fall
TORONTO, Jan 18 (Reuters) - Canada's main stock index fell on Monday as energy shares exerted further pressure on the overall index after crude oil prices hit 2003 lows, while Suncor Energy Inc slipped after upping its offer for Canadian Oil Sands Ltd.
The pair of oil companies were among the most influential movers on the index, with Canadian Oil Sands surging 12 percent to C$8.38 and Suncor off 3.8 percent at C$30.04.
Another major producer, Canadian Natural Resources Ltd , declined 2.3 percent to C$23.88. The overall energy group retreated 2.1 percent, and has lost almost 30 percent of its value since October.
At 10:07 a.m. EST (1507 GMT), the Toronto Stock Exchange's S&P/TSX composite index was down 63.99 points, or 0.53 percent, at 12,009.47.
Of the index's 10 main groups, only financials were in positive territory, gaining 0.3 percent, with Toronto-Dominion Bank advancing 1 percent to C$49.95.
The materials group, which includes precious and base metals miners and fertilizer companies, lost 0.8 percent.
U.S. markets are closed for the Martin Luther King Jr. holiday, while an attempted rebound in European stocks fizzled.
Brent oil fell below $28 a barrel for the first time since December 2003 after international sanctions against Iran were lifted over the weekend, allowing Tehran to return to an already over-supplied oil market.
U.S. crude was down 1.5 percent to $28.98 a barrel, while Brent crude lost 1.1 percent to $28.62.
Decliners were outnumbering advancers by more than 4-to-1 on the TSX, with 19 stocks hitting fresh 52-week lows. (Reporting by Alastair Sharp; Editing by Paul Simao)