MOUNT ST. HELENS NATIONAL MONUMENT, Wash. – As scientists warned that an eruption of Mount St. Helens appeared imminent yesterday, eager tourists camped out along park roads, hoping to catch a glimpse of the seething volcano without being overcome by ash and smoke.

A second long tremor early yesterday and an increase in volcanic gases strongly suggest magma is moving inside, researchers from the U.S. Geological Survey said. The mountain’s alert was raised to Level 3, the highest possible, after a volcanic tremor was detected Saturday for the first time since before the mountain’s 1980 eruption.

“I don’t think anyone now thinks this will stop with steam explosions,” geologist Willie Scott said yesterday at the Geological Survey’s Cascades Volcano Observatory in Vancouver, Wash., about 50 miles south.

At this point, scientists do not expect anything close to the devastation of the May 18, 1980, explosion, which killed 57 people and coated much of the Northwest with ash.

“Of course the volcano reserves the right to change its mind,” said monument scientist Peter Frenzen with the U.S. Forest Service, which operates the park.

Some experts had said Saturday that an explosion would probably happen within 24 hours. But as the hours passed, others cautioned that the timing is difficult to predict.

“No one is predicting it as a sure thing,” said Bill Steele at the University of Washington’s seismology lab in Seattle. “This could be going on for weeks.”

Crowds gathered at what was said to be a safe distance – about 8.5 miles from the mountain – to see what happens next. On Saturday, hundreds of people were cleared from a popular tourist observatory following a tremor and brief release of steam.

At the Coldwater Ridge Visitors Center, the wraparound veranda was jammed with people in lawn chairs, almost all with cameras at the ready.

“It’s beyond amazing,” said Steven Uhl, 31, a self-described volcano nut from Everett. “Just to be here is almost a religious experience.”

At midday, the mountain was outwardly quiet. Clouds of dust rose occasionally, caused by rock fall from the towering canyon walls. But earthquakes were occurring “multiple times per minute,” Mr. Steele said, peaking every few minutes at magnitudes as high as 3.

Scientists were unsure how explosive the eruption may be; depending on the gas content of the magma and conditions, it could range from a passive emission to an explosion that throws up a column of ash, Mr. Scott said.

Besides lava flows, ash, and rock throwing, an eruption could cause melting of the volcano’s 600-foot-deep glacier and trigger debris flows to the barren pumice plain at the foot of the mountain.

The 1980 blast obliterated the top 1,300 feet of the volcano, devastated miles of forest and buried the North Fork of the Toutle River in debris and ash as much as 600 feet deep.

This time, scientists expected populated areas to get little ash if the light west-northwest wind holds. The closest community is Toutle, 30 miles west near the entrance to the park in the Gifford Pinchot National Forest, about 100 miles south of Seattle.

The main concern was a significant ash plume carrying gritty pulverized rock and silica that could damage aircraft engines and the surfaces of cars and homes.

The mountain took scientists on a “rollercoaster ride” early yesterday when instruments detected the second extended volcanic vibration in two days – 25 minutes long compared to Saturday’s 50-minute vibration.

“It died off and quickly became a nonissue. But had it been as long as the one following that little steam burst yesterday, we could be moving to an eruption pretty quickly,” Mr. Steele said.

Scientists also detected elevated levels of carbon dioxide and other volcanic gases that reflect changes in the volume of magma rising within the mountain. Scientists at the rim of the volcano smelled hydrogen sulfide, similar to rotten eggs, said Jeff Wynn, chief scientist for volcano hazards at the U.S. Geological Survey’s Cascade Volcano Observatory.

Gas-sampling flights continued yesterday, and acoustic equipment had been placed around the crater. Dozens of Global Positioning System stations – to alert scientists to changes in ground formation – have been placed on the mountain, though Friday’s steam blast destroyed equipment on the 1,000-foot lava dome.

Most of the action has occurred beneath the dome, which has been building up on the crater floor and essentially serves as a plug for magma, or molten rock. The dome is filled with lava that came up during 1998 earthquakes but never surfaced. New lava may be coming up as well.