Demonstrating the spirit of cooperation that helped win them their third All-American city designation, Rockville residents combined talents at t a festival Sunday that included an effort to knock down the world's lap sitting record.

The lap-sitting stunt was only one attraction as about 2,000 of the young and old of Rockville frolicked on the courthouse square patio and fields adjoining Rockville Mall. They were celebrating their All-American title, awarded by the National Municipal League.

The celebrants also were entertained by more conventional means: bands, puppeteers, pantomimists, singers and dancers, while partaking of popcorn and soda, courtesy of the city government.

The verdict on the lap sitting awaits word from the Guinness Book of World Records people, who have been sent an aerial photograph of Rockville's lap sitters (the record is 1,817 persons). But the judgement on the city's community efforts had already been rendered by the Municipal League. After winning the award in 1954 and 1961, Rockville again was named All-American for 1976-77 for its citizens' achievements in community development. It is one of only two cities to receive the designation three times during the 29-year history of the competition.

In a brief ceremony at the festival Rockville Mayor William E. Hanna, Jr. accepted the All-American City award from a representative of the League, Franklin P. Hall.

A telegraph of congratulations from President Jimmy Carter and a proclamation heralding the city's achievement from the Montgomery County Council also marked the event.

What makes Rockville All-American? The League cited the city's bi-weekly newsletter delivered to all households, a beautification project and citizen leadership in directing the spending of federal funds on the city.

The city's alternative community service commission, established to combat delinquency, was given special note.That program - run by citizens - handles cases such as disorderly conduct, purse snatching and drunkenness in parks.

The offender has an opportunity to appear before the commission, which assigns a task in the community instead of sending him to jail. If the work requirement is met, the offender's record is expunged without him ever having appeared before a judge.

Rockville City Police Chief Charles Wall is enthusiastic about the commission's work, which began a year ago. So far, he said, those who appeared before the commission instead of in court have not become repeat offenders.

Wall says the citizens have a good working relationship with the 26-member Rockville police force.

"We're not a city of angels," Wall said. "We have our bad, but when we're in a place, we're there because the people want us."

Hanna echoed the police chief's civic pride. He said Rockville has retained its small town image while standing in the shadow of a major metropolis.

"There isn't a day I can't walk down the street and see someone I know," Hanna said.

Rockville, which has a population of about 50,000 is the second largest city in Maryland. The first is Baltimore.

About 10 per cent of the population are members of minority groups.