The explosion hurled Christopher Lyon nearly 30 feet to the ground at Triram at 721 Waverley St.

Lyon, of Brooklyn, Conn., was taken by rescue helicopter to Massachusetts General Hospital in Boston where he was listed in critical condition, authorities said.

A second worker, whose name was not released, was also injured, although less seriously. He was taken by ambulance to MetroWest Medical Center in Framingham. His condition last night was not immediately available.

A Framingham firefighter, whose name was unavailable, was treated at the medical center for heat exhaustion, Fire Chief Michael Smith said.

The 1 p.m. explosion shook homes and businesses in the area, frightening neighbors of the large asphalt plant that has been in operation for 45 years.

Firefighters rushed to the factory near Cedar Street and Waverley Court and called for help from the Ashland and Hopkinton departments. More than 20 firefighters used a chemical foam to fight the blaze for more than an hour before it was under control.

"When we arrived, the tank was fully involved," said Smith. "We had very serious concerns that the other (six) tanks would light up."

At one point, water used to keep the 15,000-gallon tank cool stopped flowing, and asphalt boiled over the lid and the building's roof, forcing spectators, police officers and firefighters to flee.

"Once it gets hot, it's like any other flammable material," said Smith. "It was a tough fire. It's hard to get to once it gets going."

Waverley Street, between Fountain and Winthrop streets, was blocked off for several hours, as dozens of fire, police and public works vehicles prowled the area. Puddles of water, foam and asphalt covered the parking lot.

Neighbors said they felt the ground shake when the tank exploded.

"I felt it all the way out at my house," said Raymond Howland, who lives on Bridges Street, about a half-mile away.

Howland and a friend, James Antonio, were watching police and firefighters battle flames and smoke from Antonio's back yard following the explosion.

"I was watching TV at home," said Howland. "It shook the house. I didn't know if a truck hit a (utility) pole or what."

Company manager Jim Porcello, who had gone out on an errand, returned to find the inferno, which blackened the sky with smoke.

"I'm in shock," said Porcello, who has worked at the company for more than 20 years. "I went to run an errand, and when I came back traffic was blocked. I saw smoke, and I said, 'That can't be us,' and I ran down here. Jesus, I don't know what happened. I feel so (expletive deleted) helpless. You can repair the damage, but you can't repair a life."

The welders had been working at the plant for the past eight or nine weeks, connecting new pipes to the large tanks, he said. There had never been such an incident in the company's history, he said.

Chief Smith said the town had not sanctioned the welding work. Such work would have required municipal permits, he said.

"We would have set some conditions," said the chief. "We probably would have had them transfer the material to another tank before they started welding. It would have had to be empty."

Waverly Market owner David Stefanini said welder Lyon had become a good customer over the past several weeks.

Earlier this week, Lyon had asked him to set aside a box of his homemade pastries. He planned to pick them up after work yesterday, he said.

Investigating the blaze are the local department, the state fire marshal's office, the local wiring inspector and the federal Occupational Safety and Health Administration.

Ashland firefighter Peter Chisholm praised the work of Framingham firefighters.

"They made a tremendous stop here," said Chisholm. "It's a credit to the department, it really is. This could have been a disaster here if the other tanks went up."

(News Staff Writer Peter Reuell contributed to this report.)