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 is a 12-year-old girl who is believed to be sheltering under a table, but rescuers say they do not know how to reach her.
At least 21 children and five adults died as the school collapsed.
The school was one of dozens of buildings toppled by the quake. So far 230 people are known to have died.
President Enrique Peña Nieto has declared three days of mourning for the victims.
As rescue operations continued for a second night, attention was focused on the Enrique Rébsamen 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.
"They are alive! Alive!" he shouted.
"Someone hit a wall several times in one place, and in another there was a response to light signals with a lamp," he added.
One mother, standing nearby waiting for news of her seven-year-old daughter, told reporters: "No-one can possibly imagine the pain I'm in right now."
Later Jose Luis Vergara, who is co-ordinating the operation in the school, told local TV: "We know that there is a little girl alive inside but we don't know how to reach her... without provoking a collapse or endangering personnel."
Rescuers said they were able to speak to the girl, who said she was "very tired", adding that there were other children nearby but she could not tell whether they were alive. She was also given water and oxygen.
An unnamed rescuer told AFP news agency he had seen five children alive but that they were trapped by metallic rods and the job of cutting them without hurting the children was "very delicate".
See how a Mexico City street looked before and after the quake
20 September 2017
At the scene: Efforts focus on trapped child
Paul Blake and Tona Cebada, BBC News, Mexico City
Enrique Rébsamen school was brimming with hundreds of first responders, soldiers, doctors and even teachers 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, shouting orders over a tannoy.
At one point, a rescuer emerged from the rubble with a frantic query - there was a girl trapped, but alive, on what was a lower floor. Should rescuers focus efforts on her or continue searching other parts of the collapsed structure?
As anxious families looked on, the military officer shouted that rescuers should only focus on the trapped girl, and other areas within the structure would have to wait. Scores of workers sprang into action, labouring for hours to try to free the girl.
As night fell over Mexico City a steady rain began to fall. Search operations were slowed as workers spread tarpaulins over the rescue site. With the possibility that lives can still be saved, rescuers were battling to save any souls left beneath the rubble.
Waiting crowds were frequently hushed as workers listened for sounds of life under the rubble.
Eleven children and one teacher have so far been rescued.
More than 500 members of the army and navy together with 200 police officers and volunteers have been working at the site.
According to Reuters news agency, the country's elite team of rescuers, known as "the moles", are leading the volunteer rescue efforts. The group was formed in the wake of the devastating 1985 earthquake that killed up to 10,000 people.
Mexico City authorities kept hopes alive on Wednesday, saying 52 people had been rescued so far from collapsed buildings.
Mayor Miguel Angel Mancera said 39 buildings in the capital had crumbled and search teams were scouring nearly all of them for survivors.
Most of the victims of the 7.1 magnitude earthquake died in the capital, according to government figures:
- Mexico City: 100 dead
- Morelos state: 69 dead
- Puebla state: 43 dead
- Mexico state: 13 dead
- Guerrero: Four dead
- Oaxaca: One dead
Meanwhile, police have stepped up patrols in areas hit by the quake to prevent vandalism and looting.
Mexico is no stranger to earthquakes and earlier this month an 8.1 magnitude tremor in the south left at least 90 people dead.
Tuesday's tremor struck shortly after many people had taken part in an earthquake drill on the 32nd anniversary of a major quake that killed thousands in the capital.
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