* TSX down 108.50 points, or 0.87 percent, at 12,398.56 * Eight of 10 sectors weaker, led by banks, resources By Claire Sibonney TORONTO, April 3 (Reuters) - Toronto's resource-heavy main stock index fell sharply on Tuesday, hurt by soft commodity prices and a drop in shares of Royal Bank of Canada, which U.S. regulators have accused of running a trading scheme "of massive proportion." Canada's biggest bank was the most influential decliner, down 2.8 percent to C$57.10 after the Commodity Futures Trading Commission filed a civil lawsuit against it on Monday. RBC has responded, calling the allegations "absurd." "Like everything people will take 'em out and shoot 'em until they find out what the actual true story here is. At the end of the day, there may be a fine involved, may not be," said Barry Schwartz, vice president and portfolio manager at Baskin Financial Services. "No question it's pulling down the whole bank sector." Financials were down 1.1 percent, materials were off 1.3 percent and energy shares lost 1 percent. At 10:18 a.m. (1418 GMT), the Toronto Stock Exchange's S&P/TSX composite index was down 108.50 points, or 0.87 percent, at 12,398.56. Eight of the 10 main groups were in negative territory. Other heavy laggards included Bank of Nova Scotia, down 1.1 percent to C$55.53, Suncor Energy, down 1.3 percent to C$32.78, and Toronto-Dominion Bank, 0.6 percent lower at C$84.02. Risk appetite was also hurt by weakness in oil and metal prices amid concern on Tuesday that the euro zone remained vulnerable to Spain's financial struggles, eclipsing recent enthusiasm following better U.S. and Chinese manufacturing data in the previous session. "Commodities are probably reacting to continued fears of recession in Europe and how that's going to work its way to China," added Schwartz.