MIAMI BEACH, OCT. 28 -- Labor Secretary William Brock said today the omnibus trade bill now before Congress would cause "rampant panic" in world financial markets and should be withdrawn.

But the Reagan administration, which has been working with Congress to craft a "constructive" trade bill, quickly disassociated itself from Brock's remarks. The labor secretary spoke at an impromptu news conference at the AFL-CIO convention here.

On Capitol Hill, Democratic lawmakers, already moving cautiously on trade legislation to avoid upsetting jittery markets, attacked Brock's comments.

Aides to Senate Minority Leader Robert Dole (R-Kan.) pointed to a letter Dole and 37 other Republican senators sent to President Reagan Tuesday supporting a moderate trade bill. Brock leaves the Cabinet Sunday to run Dole's presidential campaign.

Brock, who was U.S. trade representative for four years before taking the labor post in 1985, indicated that any trade bill coming out of Congress is likely to be viewed as protectionist by world financial markets and could trigger a new wave of panic selling.

"The thought that we would run the risk of a major attack on the world trading system at a time when the world is as tumultuous, as dangerous, as panicky as it is at this moment is irrational," Brock said.

He said the "authors of the legislation should seriously consider withdrawing it and backing off. "We ought to draw back from any step that would further destabilize the world system ... and rethink what we want to do," Brock said.

The omnibus trade bill now is before a House-Senate conference. It is unclear whether the complex legislation will get out of conference by the end of the year, but the Reagan administration has been working with Congress to meld Senate and House versions into an acceptable law.

The administration is particularly anxious to get authority in the bill to negotiate in a round of trade talks to improve the compact that governs world trade, the General Agreement on Tariffs and Trade. This is seen as potentially having a positive impact on the markets by strengthening the world trading system.

Gary Holmes, a spokesman for the U.S. Trade Representative's office, said the administration has not changed its position and "continues to support a constructive trade bill and will work with Congress to achieve it."

On Capitol Hill, Senate Finance Committee Chairman Lloyd Bentsen (D-Tex.) called Brock's remarks "outrageous," and said the labor secretary was "always against the trade bill. ... Now he is seizing on the stock market to further malign it."

Dole's office cited a letter to Reagan by 38 GOP senators, headed by the Republican leader. "In light of events in the stock market," the letter said, "we want to assure you once again that we will oppose the enactment of protectionist legislation that would have adverse consequences for the United States."

At a trade bill conference last week, Bentsen, U.S. Trade Representative Clayton Yeutter and House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) emphasized that they were working together to draft a trade bill that would not be protectionist. It was clear they were trying to assure the markets.

Brock's remarks drew an immediate rebuke from the AFL-CIO, one of the major backers of the trade legislation, which overwhelmingly approved a resolution Tuesday calling for passage of the trade bill. Labor argues that more restrictive trade legislation is needed to help eliminate the nation's huge trade imbalances.

Washington Post Staff Writer Stuart Auerbach contributed to this story.