NEW YORK (AP) _ Wall Street clerical workers picketed outside the New York Stock Exchange today after voting to strike, but the exchange opened on time and officials maintained the walkout would not disrupt the crash-weary market.

''The integrity of the marketplace will be maintained,'' said Richard Torrenzano, a vice president of the exchange, Monday night after the clerical union took its strike vote.

About 400 pickets from Local 153 of the Office and Professional Employees International Union, which represents about 1,100 workers, marched in front of the Broad Street entrance to the exchange this morning.

The union voted 2-to-1 Monday to go on strike, said Michael Goodwin, the union's secretary-treasurer.

William Stockdale, the captain of the pickets who works in the exchange's security department, said the strikers would not interfere with anyone entering or leaving the building. ''We won't stop people who want to go into work,'' he said.

Stockdale said some union members had gone in to work early this morning, but he declined to estimate how many.

Torrenzano, however, estimated that 20 percent of the union workers had crossed the picket lines.

The exchange opened as scheduled at 9:30 a.m. EST, with supervisory personnel performing the jobs of the striking employees. In the only visible concession to the strike, the visitors gallery, which is staffed by union workers, was closed for the day.

Marvin Dantowitz, who has worked on the floor of the exchange for 28 years reporting sales and quotations, said he did not believe management could fill in properly.

''It takes three to five years to become a good reporter. They've been training a month ...,'' he said. ''I wouldn't rely on the accuracy or the timing of the quotations.''

''You're going to see a lot of corrections,'' added Jim Saccardo, an exchange reporter for 27 years.

The union had been working under terms of its previous contract since Oct. 31. A bargaining session with a federal mediator ended Monday afternoon without producing an agreement, largely because of the exchange's position on pensions, Goodwin said.

The union represents floor reporters, clerks, secretaries and maintenance workers at the exchange and about 325 clerical workers at the Securities Industry Automation Corp., which handles securities transactions.

Torrenzano said the exchange had contingency plans for a strike and said it would ''function in an orderly manner throughout the day.''

Goodwin said the last strike by clerical workers at the exchange was about 40 years ago, but he did not know details of that walkout.

The exchange's final offer Monday included wage increases of 5 percent in each of the next three years, as well as an additional floating holiday, improvements in the pension and savings plan and other benefits, Torrenzano said.

''This offer is reasonable and serious,'' Torrenzano said.

Goodwin said the pressures on floor reporters, who mark the stock trades and enter them into a computer, make improvements in pensions necessary.

''If you've watched the market the past few weeks, it's easy to see how a worker can burn out readily, handling those deals on the floor,'' he said. ''With all of the activity and volume, these workers have to keep up.''

The union is asking for a pension of half-pay at age 55, he said. Currently, a worker earning $33,700 a year with 35 years' service who retires at 65 would receive a pension of $11,700. The exchange, he said, offered to move retirement up to age 62 and improve early-retirement incentives.

''We believe the union's position is far from realistic,'' Torrenzano said.

According to Goodwin, the exchange feels its savings plan sufficiently supplements the pension, but Goodwin said employees contribute two-thirds of the money that goes into the savings plan.

Torrenzano said a trading floor reporter works 33 hours a week and earns about $36,500 a year. ''So we think these people are well paid,'' he said.

Goodwin said, ''Outside of the pension issue, the balance of the package is essentially acceptable.''