"A policeman's lot is not a happy one," sang the constabulary chorus on the National Theatre stage, where "The Pirates of Penzance" opened last night. The musical's producer, Joseph Papp, agreed--with a slight amendment: "May the policeman's lot be an unhappy one," he said as he arrived for the black-tie, opening night party that he staged at Georgetown's Four Seasons Hotel.

Papp was referring to his day, which he had spent in New York City--picketing, being arrested and booked for criminal trespass, and then watching the wrecker's ball swing against two landmark Broadway theaters. After all that activity, he caught the 6 p.m. plane to Washington to be on hand for the "Pirates" opening.

"You know one of the cops almost cried when he arrested us," Papp said. "And he was tough as nails. I got to know him pretty well after days of picketing. The cops were really quite charming to us."

Papp and an all-star cast of about 200 Broadway stars and theatrical people were arrested yesterday as they picketed in front of the Morosco and Helen Hayes theaters, which are now being razed to clear the way for the 50-story Portman Hotel. The Supreme Court cleared the way for the demolition yesterday when it lifted a temporary stay that had been issued by Justice Thurgood Marshall.

"It was a big party," Papp said, smiling, last night as he sipped a drink and recalled the day's drama. "We had a hundred more who wanted to be arrested after we filled 11 vans. Gail, my wife here, was also arrested. We were cellmates."

After his booking at the police precinct, Papp went back to the site of the theaters and stayed until the wrecker's ball swung into the Morosco a little before 5 p.m.

"It bit a gnash into the wall. People screamed as if a baby was being thrown out of a window. We were covered with grime and soot."

The producers' party in Georgetown mixed guests from the government, embassies and the media with the theatrical crowd.

The crowd included Michael Deaver, White House deputy chief of staff; Larry Speakes, deputy White House press secretary; Rep. James C. Wright (D-Texas); Sen. Christopher Dodd (D-Conn.); Peter Hannaford, the public relations man; and Cathy Douglas, widow of the Supreme Court justice. Novelist William Styron made a special trip from Connecticut to see his goddaughter, Caroline Peyton, who plays the role of Mabel, the ingenue lead.

The members of the cast of the National's "Pirates" were the stars of the party.

James Belushi, younger brother of actor John Belushi who died earlier this month in Hollywood of an apparent drug overdose, came to the party from his role as the Pirate King in the production, which trouped from Chicago to Washington.

"It's better than a Turkish bath," said Belushi of his role. A robust man, he turns in several acrobatic tricks as the Pirate King.

Belushi, who arrived in black-tie with his wife, Sandi, was embracing some of the maids in the chorus as he held onto a bottle of white wine.

"I play the role very physical, from my appearances at Second City in Chicago where audiences respond to that," he said.

The younger Belushi had asked that he not be questioned about his brother's death. A friend said John Belushi had hoped to see his younger brother in the stage role in Chicago.

Among those in the audience were three members of the New York company who took a busman's holiday on their night off to see the road company's performance.

They included George Rose, who is the Major-General in the New York production, Patrick Cassidy, son of the late star Jack Cassidy, who plays Frederic on Broadway, and Maureen McGovern, who is playing New York Mabel, originated on Broadway by Linda Ronstadt.

For those arriving at the hotel for the post-theater buffet, Papp had arranged some realistic stage effects, including a heavy fog, as the guests strode over the gangplank entrance.

"I think we're getting fumigated," said one guest.