Three securities firms and 21 brokers have been charged with fraud, authorities said, accused of a price-rigging scheme that caused thousands of stock market investors to lose more than $10 million.

Manhattan District Attorney Robert Morgenthau said Wednesday that from September 1987 through February 1990 a group of "market makers" manipulated prices of over-the-counter stocks in certain companies by buying or selling them over and over again within the group.Apparent demand for a stock drives up its price, allowing members of the alleged conspiracy to sell for an artificially high price. Similarly, lack of demand drives down the price, allowing an artificially low purchase price.