(Releads with details of accident)
By Mathieu Belanger
LAC-MEGANTIC, Quebec, July 6 (Reuters) - A driverless freight train carrying tankers of petroleum products derailed at high speed and exploded into a giant fireball in the middle of a small Canadian town early on Saturday, destroying dozens of buildings and leaving an unknown number of people feared missing.
The disaster occurred shortly after 1 a.m. (0500 GMT) when the runaway train with 73 cars sped into Lac-Megantic, a picturesque lakeside town of about 6,000 people near the border with Maine, and came off the rails. Witnesses said the town center was crowded at the time.
Four of the pressurized tank cars caught fire and blew up in a fireball that mushroomed many hundreds of feet up into the air.
Although police said they could not yet get close enough to determine the number of victims from the still-burning fires, an aerial photograph showed widespread devastation in the town center. Police also said that some in the town were worried because they could not reach missing family or friends by phone.
An official from Montreal, Maine & Atlantic, the firm that operated the train, said the train had been parked some distance away from the town and no one on board when it derailed.
"We're not sure what happened, but the engineer did everything by the book. He had parked the train and was waiting for his relief ... somehow, the train got released," vice president of marketing Joseph R. McGonigle told Reuters.
"There appears to be extensive damage in buildings, but we haven't got full report yet of injuries. But we understand that there likely are some," he said.
The rail tracks pass next to a bar popular with young people. Eyewitness Yvon Rosa said he had just left the bar when he saw the train speeding into the middle of the town.
"I have never seen a train traveling that quickly into the center of Lac-Megantic," he told French-language broadcaster Radio-Canada, saying he watched as the train hurtled around a bend.
"I saw the wagons come off the tracks ... everything exploded. In just one minute the center of the town was covered in fire."
Residents told reporters they had heard five or six large blasts. Nearly 12 hours after the derailment, one rail car was still burning.
"Many parents are worried because they haven't been able to communicate with a member of their family or an acquaintance," Mayor Colette Roy-Laroche told Radio-Canada.
"We can't give out any information on what's happening right now because the firemen haven't been able to get close."
Fire officials said they feared more of the tanker cars were at risk of exploding. Around 30 buildings in the town center were destroyed, some by the initial blast and others by the subsequent fire, they said.
Police imposed a 1/2-mile (1-km) security zone around the blast and evacuated about 1,000 people from their homes.
"When you see the center of your town almost destroyed, you'll understand that we're asking ourselves how we are going to get through this event," a tearful Roy-Laroche told a televised news briefing.
Prime Minister Stephen Harper made an initial statement via Twitter: "Thoughts & prayers are with those impacted in Lac Megantic. Horrible news."
Lac-Megantic is part of Quebec's Eastern Townships region, an area popular with tourists that is close to the border with Maine and Vermont. Quebec is a predominantly French-speaking province in the eastern half of Canada.
Fire officials said they had asked for help from fire services in the United States. Around 20 fire engines were fighting the blaze.
Police said some of the tanker cars had spilled their contents into the river that runs through the town.
"I can say absolutely nothing about victims... We've been told about people who are not answering their phones, but you have to understand that there are people who are out of town and on holiday," police spokesman Michel Brunet told the briefing.
Montreal, Maine & Atlantic owns some 510 miles (820 km) of track in Maine and Vermont in the United States and in Quebec and New Brunswick in Canada.
Fire officials said they suspected some of the remaining tanks could still be pressurized.
"We're not sure because we can't get close, so we're working on the assumption that all the cars were pressurized and could explode. That's why progress is slow and tough," said local fire chief Denis Lauzon.
Satellite images show the railroad snaking through downtown, following a route parallel to the shore of the lake that the town is named after, before heading inland and crossing the U.S. border to the west of town.
There have been a number of high-profile derailments of trains carrying petroleum products in Canada recently, including one in Calgary, Alberta, last week when a flood-damaged bridge sagged toward the still-swollen Bow River. The derailed rail cars were removed without spilling their cargo. (Reporting by David Ljunggren in Ottawa, P.J. Huffstutter in Chicago,; Editing by Eric Beech and Jackie Frank)