The Soufriere Hills volcano that destroyed this Caribbean island's capital in 1997 shot a cloud of ash more than five miles (eight kilometers) into the sky, prompting evacuation orders for some homes.
With more volcanic activity likely, Montserrat's British governor ordered the evacuations and said police would enforce the decision.
Monday's blast, accompanied by increased seismic rumbling, released gases and steam from inside a lava dome that has grown rapidly over the last week, said Dr. Vicky Hards, director of the Montserrat Volcano Observatory.
"I think it was a warning call ... of what it can do," Hards said.
The explosion around sunrise also sent a flow of volcanic material cascading three kilometers (two miles) down the volcano's northwest flank, but did not threaten any of the British island's 5,000 inhabitants, Hards said. Sirens alerted people to listen to the radio for updates.
The government has advised about 50 households on the northwestern side of the volcano's base that their homes would be at risk from blistering gas and debris if the dome collapses. Officials conducted door-to-door briefings in the low-lying Belham Valley over the weekend.
Gov. Deborah Barnes Jones signed an evacuation order making it illegal for people to remain in that area as of Monday evening.
"People in the affected area know who they are and should work urgently on packing up and arranging for alternative accommodations," she said in a radio address.
"It will be an offense to be in the unsafe zone, and the police will prosecute offenders," Barnes Jones added.
Wind blowing from the east pushed the dark gray ash over the "exclusion zone," a barren, uninhabited area extending from the 3,000-foot (900-meter) high volcano across the southwest to the coast. Open water lies west of the island.
A hotel located near the exclusion zone has already emptied, and only "a handful" of residents were believed to still be living in the threatened area, said Mark Twigg, head of the governor's office.
"This causes genuine hardship for people who have to leave, and this is taken lightly by nobody," he said.
The volcano's latest burst of activity began on Dec. 24. Glowing streaks of red from the pyroclastic flows have created nighttime spectacles visible across much of the island. The volcano's rising dome remained in place after Monday's explosion, raising fears of a bigger event soon.
"The flows also could have opened a line to go farther down the valley," Twigg said.
The Soufriere Hills volcano became active in 1995, and more than half the territory's 12,000 inhabitants moved away. An eruption in 1997 buried much of the south, including the capital of Plymouth, and killed 19 people, the AP says.
Since then, the mountainous, teardrop-shaped island has gone on a building binge. A new city center is planned for Little Bay, the future capital, in northwest Montserrat. The island has a new airport to replace the one that was engulfed by volcanic flows and a 700-seat concert hall. A new parliament, courthouse and cricket field are planned.