LIVERMORE, Calif. (AP) _ About 500 people demonstrated at the Lawrence Livermore National Laboratory in a Good Friday anti-nuclear protest, and 40 were arrested, mostly on misdemeanor charges of obstructing traffic, authorities said.

One man was arrested at the nuclear weapon lab's Site 300, about 20 miles east of the installation, for entering the property where conventional detonators for nuclear weapons are tested. He did not get near the testing site.

Four of the people who walked onto a tiny island outside the west gate of the lab and planted flowers were arrested by security police on vandalism counts.

''The flowers were very nice, actually,'' said lab spokeswoman Bonnie Jean Barringer.

No one was hurt and the protest was reported to be peaceful. The lab is about 50 miles east of San Francisco.

Witnesses said the demonstrators started gathering on Vasco Road, edging the west side of the lab site, at about 6 a.m. Some of them carried what looked like a coffin, and a large cross with a figure pinned to it.

''They appear to have been coordinated by the Livermore Action Group,'' said Ms. Barringer. The L.A.G., headquartered in Berkeley, has staged many protests at the site over the years.

Phone calls to the Livermore Action Group office went unanswered.

At about 8:15 a.m., people from the main group broke away walked onto East Avenue holding hands. They were arrested by the Highway Patrol and taken for processing to Santa Rita Rehabilitation Center, an Alameda County sheriff's facility.

''Most of the morning shift was already in the lab,'' said Ms. Barringer. The facility employs about 8,000 people.

The laboratory designs, manufactures and controls the testing of weapons for the U.S. nuclear arsenal, and is operated for the government by the University of California.