Rescuers are racing against the clock to reach survivors trapped under the rubble of a school in Mexico City which collapsed during Tuesday's earthquake. One of them, a 13-year-old girl, is believed to be sheltering under a table, according to the BBC, quoting officials at the scene.

At least 21 children and five adults are thought to have died when the primary school collapsed, and many others are missing.

The school was one of dozens of buildings toppled by the quake, in which, so far, 230 people are known to have died.

President Enrique Pena Nieto has declared three days of mourning for the victims.

As rescue operations continued for a second night, attention was focused on the Enrique R?bsamen primary school, in Mexico City's southern Coapa district. With anxious parents gathered outside awaiting news of missing children, civil protection volunteer Enrique Gardia announced that a thermal scanner had detected survivors trapped between slabs of concrete.

"Someone hit a wall several times in one place, and in another there was a response to light signals with a lamp," he said.

Hundreds of first responders, soldiers, doctors and even teachers are racing against the clock to rescue people believed to still be alive beneath the collapsed classrooms. From the roof of a school building opposite the classrooms, Mexico's Secretary of Public Education and a senior military officer were commanding the rescue operation.

Mexico City authorities kept hopes alive on Wednesday, saying that 52 people had been rescued so far from collapsed buildings.

The BBC report quoted Mayor Miguel Angel Mancera as saying that 39 buildings in the capital had crumbled and search teams were scouring nearly all of them for survivors.

President Nieto, visiting the damaged city of Jojutla in Morelos state, appealed for citizens to help rescue operations. "It is important for people to be part of this group effort," he said.

Meanwhile, police have stepped up patrols in areas hit by the quake to prevent vandalism and looting and schools in the affected states have suspended classes until Monday and all professional football fixtures have been cancelled for the weekend.