Mayor Marion Barry announced last night that he will modify the District's controversial new regulations for sidewalk vendors and delay enforcement of new rules for 15 more days.

Barry's action, which came in response to repeated complaints by vendors that the rules are too restrictive, was met with cautious enthusiasm by the sidewalk sales force but was criticized severely by business groups.

Donald Slatton, an executive officer of the Apartment and Office Building Association, said that his organization had worked with the public commission that helped to developed the new standards, spent $11,000 to help the city successfully fight a lawsuit filed by the vendors to block implementation of the rules and had been "cut off at the knees" by Barry's decision.

"I am absolutely livid," said Slatton. "I accuse the mayor of bowing to political pressure. We just think this is the last nail in the coffin. This administration is anti-business."

The new rules went into effect on May 15 but the city had agreed not to begin enforcing them until tommorrow. Yesterday, the mayor extended the enforcement date until June 30 to give the city time to prepare modified rules.

Members of a group of 200 vendors who organized to oppose the regulations, had requested some of the proposed modifications, which address some of the most controversial regulations. Barry plans to lift some of the restrictions that placed a ban on the sale of most clothing items and other goods readily available in stores, alter the requirement that all nonfood vendors display their wares on a wooden cart and establish a 11-member vending advisory commission.

"I am committed to maintaining a balanced approach to vending in our city," Barry said in a statement released last night. "I also believe that the city must develop design standards that are aesthetically pleasing yet allow the vendors to easily transport their merchandise . . . . "

Barry said that new requirements for displaying vending merchandise must be developed and will be implemented by Oct. 1.

Joan Comedy, who has been a vendor since 1973 and attended a briefing on the mayor's modifications, said that vendors were pleased by some of the changes but would need to discuss the recommendations before accepting them.

But a spokesman for the Greater Washington Area Board of Trade said the mayor's action appears to be "a retreat from the reforms developed" during the last two years and that the board is "surprised and distressed."

The city developed the new vending rules to reduce sidewalk congestion, protect consumers and improve sales tax collections. Vendors, however, argued that the rules were an effort by the bigger businesses to stifle low-cost competition. They met with Barry on Tuesday to ask him to modify the rules.

"I recognize that vendors are an important segment of our business community," Barry said yesterday, "and we will continue to work with them and other members of the business community to establish balanced policies that will encourage the continued growth and development of businesses in our city."

Barry said that a number of changes contained in the new regulations would be retained, including higher vendor fees, restrictions on locations and requiring all vendors to cease activities by 10 p.m. on weekdays and 1 a.m. on weekends.