A state of emergency has been declared in several towns on Mexico's Pacific coast, where a powerful earthquake hit overnight, killing at least 23 people.

The hardest-hit area is the state of Colima, where many homes and walls simply crumbled because of the shock waves.

The house fell on me and my son pulled me out Virginia Lomeli Colima resident

Rescuers are searching through rubble for survivors, and hundreds of soldiers have been sent to try to reach isolated communities in the mountainous interior of the state.

The United States seismological service said there could be substantial damage, due to the tremor's size and closeness to the Earth's surface.

Mexican officials put the quake's magnitude at 7.6, while the US Geological Survey calculated it at 7.8

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At least 21 people have died in Colima, and two others in the neighbouring state of Jalisco, according to the Mexican Red Cross.

"Colima hospital is full," the state's director of civil protection, Melchor Urzua Quiroz, told the BBC. He said at least 150 people were known to have been injured.

Lost husband

The quake struck at 2000 local time on Tuesday evening (0200 GMT Wednesday), flattening buildings.

"The house fell on me and my son pulled me out. He was shouting at me and pulling me out," Colima resident Virginia Lomeli told Reuters.

Other people were still desperately searching for relatives in Colima, a colonial city of some 200,000 residents.

Alma Edith Arceo told Reuters she last saw her husband, a taxi driver, two hours before the quake struck, shaking her own home.

"I heard a loud creak, as if rocks were moving, and it felt like the house was coming down on top of me," she said.

"I am looking for help to find my husband. I haven't heard anything from him."

Strong tremors were also felt in Mexico City 500 kilometres (300 miles) away from the epicentre, reviving memories of the 1985 earthquake that buried thousands.

But the capital seems to have been spared any serious damage.

Rescue challenge

As daylight allowed rescuers to assess the scale of their task, Colima's Governor, Fernando Moreno, said help was on its way from the federal government.

Colima residents begin the clean-up

Work was also under way to restore local utilities and public services.

President Vicente Fox ordered troops to assess damage near the quake's epicentre, a region that includes remote coastal villages in Jalisco and Colima.

The BBC's Mexico correspondent, Nick Miles, says the authorities are keen to demonstrate their ability to mount a rapid rescue effort, after previous governments were criticised for inadequate responses to natural disasters.

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