Even before the opening bell, Monday looked ugly.

But by the time that bell sounded again on the New York Stock Exchange, six and a half frantic hours later, $1.2 trillion had vanished from the United States stock market.

What had started 24 hours earlier, with a modest sell-off in stock markets in Asia, had turned into Wall Street’s blackest day since the 1987 crash. The broad market, as measured by the Standard & Poor’s 500-stock index, plunged almost 9 percent, its third-biggest decline since World War II. The Dow Jones industrial average fell nearly 778 points, or 6.98 percent, to 10,365.45.

Across Wall Street, no one could quite believe what was happening on the floor — the floor of the House of Representatives, not the New York Exchange.

As lawmakers began to vote on a $700 billion rescue for financial institutions, the Voyageur Asset Management trading desk in Chicago went silent. Money managers gaped at a television screen carrying news that seemed unthinkable: the bill was not going to pass. Shortly after 1:30 p.m., the rescue was rejected.