The initial blast caused a series of fires in residential areas nearby. A nursing home is said to have been destroyed
Local residents said the devastation was 'like a war zone' and a wide area has been evacuated amid fears of further explosions
The plant is believed to have large stores of ammonia, an explosive and toxic chemical. As well as concerns about danger to further storage tanks on the site, the authorities are worried that the gas could be blown into residential areas
Around 45 firefighters are said to be unaccounted for
West Mayor Tommy Muska said: "It was like a nuclear bomb went off,"
At least two emergency services personnel are confirmed dead
Several firefighters are said to be unaccounted for following the blast.
Texas Governor Rick Perry said state officials are waiting for full details about the extent of the damage. "We are monitoring developments and gathering information as details continue to emerge about this incident," he said in a statement.
Aerial footage showed fires smouldering in the ruins of the plant and in several surrounding buildings.
A floodlit local football field was turned into a staging area for emergency crews, and dozens of people could be seen receiving treatment there.
Local resident Debby Marak described the scene as "like being in a tornado".
"Stuff was flying everywhere. It blew out my windshield. It was like the whole earth shook," she added.
It is believed that buildings in a huge area around the blast scene have been completely flattened by the force of the explosion.
West Mayor Tommy Muska told reporters that his city of about 2,800 residents needs "your prayers".
He added: "We've got a lot of people who are hurt, and there's a lot of people, I'm sure, who aren't gonna be here tomorrow. We're gonna search for everybody. We're gonna make sure everybody's accounted for. That's the most important thing right now."
A member of the city council, Al Vanek, said there is a four-block area around the explosion "that is totally decimated".
He said the damage was comparable to the destruction caused by the 1995 bomb blast that destroyed the Murrah Federal Building in Oklahoma City.
The fire is now under control, but residents have been urged to stay indoors because of the threat of more explosions.
Erick Perez, 21, said he was playing basketball at a nearby school at the time. The blast threw him to the ground and showered the area with hot embers, shrapnel and debris.
A spokesman for the Hillcrest Baptist Medical Centre in Waco told CNN his hospital had received 66 injured people for treatment, including 38 who were seriously hurt.
The American Red Cross said it is sending crews from across Texas to the scene to help the injured and those whose homes have been destroyed.
Officials from the Texas Commission for Environmental Quality, the state's emergency management department and an incident management team are on scene. Also present are the state's top urban search and rescue team, the health department and mobile medical units.
West Mayor Tommy Muska says there were five or six volunteer firefighters battling the blaze at the time of the blast. He himself is a volunteer.
He said the blast knocked his helmet off and shattered the windows of his nearby home.
He added that not all of his fellow firefighters are accounted for.
The explosion has echoes of a 2001 blast at a chemical and fertiliser plant in Toulouse, France. That blast killed 31 people and injured more than 2,000.
Country and music legend Willie Nelson grew up near where the blast occurred, and he has tweeted that his thoughts are with the town.
There is still no word on the number of casualties, but officials have confirmed there are fatalities.