Officials say at least 170 people have been killed and thousands injured, but it is feared the death toll may rise into the thousands.

The quake hit the west coast near the resort town of Acapulco early this morning, and was measured by the US Geological Survey at a magnitude of 7.8.

It lasted for 50 seconds, and devastated three states on the Pacific coast.

"A part of a mountain just slid away, falling on the peasants who were just getting up to go to work," said Lieutenant Manuel Sanchez, from the fire department's headquarters in the state of Jalisco.

Disaster zone

Most of the damage, however, was caused 250 miles (400 km) away in Mexico City, which was declared a disaster zone.

Telephone links were cut, and a communications tower burst into flames, leaving television broadcasts monitored in neighbouring Guatemala the only source of information.

Television reports said hundreds of people are trapped in rubble, and more than a third of all buildings have been damaged.

Clouds of dust hung over the city centre, and broken glass and chunks of cement littered the streets.

There was a strong smell of gas and the city government issued a radio appeal for people not to light matches.

Collapsed buildings

Several high-rise hotels collapsed entirely, as well as a section of the city's huge medical centre. Many buildings were on fire. The underground system also failed, stranding hundreds of people.

Many people gathered on street corners, several weeping and some fainting. Others joined rescue workers digging through rubble in a frantic search for survivors.

Among the dead were 25 people killed when a church near the city of Guadalajara, northwest of Mexico City, collapsed.

The President, Miguel de la Madrid, has toured the city, which has a population of about 18 million.

The government appealed for blood donors as police and troops patrolled the rubble-strewn streets to try to prevent looting.

The Mexican Embassy in Washington warned there is a risk of a major tidal wave hitting the Pacific coast from Ecuador north to California.

The quake was felt as far north as Houston, Texas, 745 miles (1,200 km) away, and in Guatemala City 621 miles (1,000 km) to the south.

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