Oklahoma tornado: Dozens killed in Moore
About 120 people are being treated in hospitals.
President Barack Obama has declared a major disaster in Oklahoma.
He also ordered federal authorities to join in the search efforts which have continued throughout the night.
It is deceptively calm; insects chirrup in the background. Everywhere is covered in a light spray of soggy debris and a film of mud.
At the deserted Moore police station, cars are spattered with mud. Larger concrete structures are still standing - a cinema, a supermarket, reminders of what there was before the tornado struck.
But houses, offices and shops have been torn apart by the force of the winds that ripped through here, sturdy telephone poles snapped at their bases, street signs sent flying.
And most disconcerting - though unsurprising - is the complete absence of any residents.
Monday's twister hit Moore, a suburb of about 55,000 people, at 15:01 (20:01 GMT) and remained on the ground for about 45 minutes.
At least 20 children were among the dead, the Oklahoma chief medical examiner's office said.
Plaza Towers Elementary school took a direct hit: the storm tore off the building's roof and knocked down walls.
"The school was flattened. The walls were pancaked in," Oklahoma's Lieutenant Governor Todd Lamb told the BBC.
"There's still roughly two dozen children that are missing. There have been some bodies recovered from that school and it's absolutely horrific and devastating."
Another school - Briarwood Elementary - was also damaged, and teachers were later seen leading pupils out to safety.
The storm destroyed several areas, leaving a tangle of ruined buildings, piles of broken wood, overturned and crushed cars, and fires in some places.
The BBC's Alastair Leithead in Oklahoma says many tornadoes in the region hit the open plain, but this one struck a residential area.
Many houses are built on hard ground without basements, so residents did not have recourse to shelter, our correspondent adds.
'School coming apart'
"We locked the cellar door once we saw it coming, it got louder and next thing you know is you see the latch coming undone," survivor Ricky Stover said.
"We couldn't reach for it and it ripped open the door and just glass and debris started slamming on us and we thought we were dead, to be honest."
James Rushing said he had hurried to Plaza Towers Elementary School, where his foster son Aiden was a pupil, to see it destroyed by the storm.
"About two minutes after I got there, the school started coming apart," he told the Associated Press news agency.