Demonstrators claiming membership in communist and veterans groups barricaded themselves inside the observation deck of the Washington Monument yesterday and hung out a banner reading, "U.S. Keep Your Bloody Hands Off Iran."

With the monument's elevator stalled by the protesters, U.S. Park Police climbed the interior stairs, scaled a fence, and stepped across an air duct to haul in the banner and end the short-lived demonstration.

Eleven persons were arrested on disorderly conduct charges following the unusual incident, which began about 4:40 p.m. and lasted about 25 minutes.

The demonstrators identified themselves as members of the Revolutionary Communist Party, known as a Maoist group, said Park Police Lt. Gary E. Treon. He said some demonstrators also claimed membership in Vietnam Veterans Against the War.

According to various police accounts, the demonstrators either smashed or cut open one of the observation deck windows, 500 feet above ground, to unfurl their green, 3-foot-by-10-foot banner with its black letters.

Police said the banner was hung from a window that faces the White House, across the Ellipse. Although President Carter was reported to be in his office at the time, it could not be learned whether he saw it.

About 10 Park Police officers were led up the normally locked 898-step interior stairway of the 555-foot-high monument by Sgts. Carl Clipper and Herman Cooper of the special operations force.

With at least one jug of water flung at the officers as they made their way over the duct 490 feet above the obelisk's bottom, the crossing was "about the hairiest" of his assignments in some time, Clipper said.

Six of the demonstrators were seized on the observation level, police said, and the other five at the ground level.

The demonstrators, several of whom wore red shirts, also affixed stickers to the monument's interior, and dropped leaflets from the top, according to one report.

Members of supporters of the Revolutionary Communist Party were arrested here earlier this year on two separae occasions connected with protests against the visit in January of Chinese Vice Premier Deng Hsiaoping.

Yesterday's demonstration, which came on Veterans Day, apparently was prompted by public duscussion of measures to win the release of hostages held at the U.S. Embassy in Iran.