Carter-Mondale campaign staffer Bruce Moran thought somebody was playing another dumb campaign stunt when he showed up for work about 9 a.m. today and saw a masked man behind the door of campaign headquarters, waving a rifle at him.

"I said, 'You've got to be kidding,' pushed the gun away and kept walking," he said afterward.

But it was no joke. An unknown number of gun-wielding members of the Puerto Rican terrorist group FALN (Armed Forces of National Liberation) took over the Carter FALN (Armed Forces of National Liberation) took over the Carter-Mondale campaign headquarters here and bound and gagged seven staff members at about the same time a George Bush campaign headquarters was similarly invaded in midtown New York, according to police and FBI officials.

Ten campaign workers were bound and gagged at the Bush headquarters. An aide in Washington said Bush had decided to accept Secret Service protection, which he had refused last January, as a result of today's incident and of an egg-throwing incident during the Puerto Rico primary campaign last February. Bush, who favors statehood for Puerto Rico, won that primary. The FALN favors independence for the island commonwealth.

Pete Teeley, a press aide in Washington, said Secret Service agents would join the Bush campaign in Illinois Monday. Bush also was protected by agents at the Gridiron dinner in Washington tonight.

The terrorists, who masked themselves in scarves, fled in both instances without injuring anyone. They spray-painted the headquarters walls with nationalist slogans and the initials of their organization, ripped out some phones and scattered papers around, according to police.

"After I got inside I saw a second man with a pump action shotgun and a woman with a handgun," Moran said. "They were very nervous. They told me to get down on my knees. One of them said, "This is a takeover.'"

Moran was bound and gagged along with his coworkers but he said he managed to untie himself as soon as the intruders left by flexing his wrists, and called for help.

It was not clear how many intruders took part in the incident, which occurred on the fifth and sixth floors of a building across from the Chicago Civic Plaza and City Hall, law enforcement officials said.

The police and Secret Service were beefing up security at the campaign headquarters of all the presidential candidates as well as other public buildings as a result of the attacks, they said.

The terrorists "got the publicity they wanted," said Jim Ingram, special FBI agent in charge of the Chicago office. The Puerto Rican nationalist group has, since 1974, taken responsibility for more than 100 bombings and the resultant death of five people, he added.

The New World Building, site of the takeover, was cleared of people and searched for explosives after the morning's events. Supporters of Rep. John B. Anderson, a GOP presidential candidate who has a campaign office in the same building, handed out Anderson buttons to the crowd on the sidewalk.

In a similar show-must-go-on spirit the Carter-Mondale staff resumed their duties as best they could between police questions.