* TSX falls 103.88 points, or 0.81 percent, to 12,735.12 * All 10 main index sectors decline * Suncor has biggest negative influence on index * Takeover target BlackBerry drops 3.6 percent * Valeant gains after drug, contact-lens approvals By John Tilak TORONTO, Oct 3 (Reuters) - Canada's main stock index dropped on Thursday to its lowest in nearly three weeks as negative sentiment caused by the extended U.S. government shutdown helped offset upbeat economic data from China. Every major sector on the index was trading in the red, taking the Toronto market into negative territory for a second straight session. Investors also cast an eye on BlackBerry, which is the focus of a $4.7 billion bid from a consortium led by Fairfax Financial. Shares of the smartphone maker shed 3.6 percent. Data showed rising demand caused activity in China's services sector to expand at the fastest pace in six months in September but failed to lift sentiment. The focus of the market remained the U.S. government shutdown, which dragged into a third day, highlighting the political gridlock in Washington over the debt crisis. "The market would suggest that they think this would ultimately get resolved," said Paul Hand, managing director at RBC Capital Markets. "But minute-by-minute people do feel uncertainty." The Toronto Stock Exchange's S&P/TSX composite index closed down 103.88 points, or 0.81 percent, at 12,735.12, after reaching 12,700.02, its lowest since Sept. 13. About 254 million shares changed hands on Thursday, compared with an average daily volume of 304 million shares in September, according to market operator TMX Group. The TSX will likely make some gains and end the year around 13,400 as the U.S. debt woes ease and sentiment for Canadian equities improves, Hand said. All of the 10 main sectors on the index were in the red on Thursday. Shares of energy producers fell 1.1 percent, weighing the most of any major sector. Suncor Energy Inc lost 1.5 percent to C$36.25 and had the biggest negative influence on the index. Canadian Natural Resources Ltd gave back 1.4 percent to C$31.78. The materials sector, which includes mining stocks, slipped 1.4 percent. Potash Corp was down 0.6 percent at C$33.69, and Teck Resources Ltd declined 2 percent to C$27.32. BlackBerry slipped to C$7.97, taking the information technology sector down 1.4 percent. But Valeant Pharmaceuticals International Inc climbed 1.4 percent to C$114.91 after the drugmaker won regulatory approval for a topical nail fungus drug in Canada and clearance for a new contact lens in the United States.