A devastating mile-wide tornado hit the US state of Oklahoma on Monday, flattening neigbourhoods, causing widespread fires and landing a direct hit on an elementary school.

Homes and buildings were reduced to rubble in Moore, south of Oklahoma City. Cars were strewn across roadways and entire blocks were flattened. The local medical examiner's office said 51 people had died, including seven children.

The National Weather Service said the storm measured an EF-4 on the enhanced Fujita scale, the second most-powerful type of tornado.

President Barack Obama declared a major disaster in Oklahoma. The state governor, Mary Fallin, deployed 80 National Guard members to assist with recovery operations as fears grew that many children would be among the dead. "Hearts are broken," Fallin told a news conference.

Hospitals were treating more than 120 people, including 70 children. Some were in a critical condition.

The storm struck in a part of the US known as Tornado Alley. Amateur video footage showed a dark funnel of cloud snaking across the landscape, flattening buildings in its path and scattering debris over the streets. Winds reached speeds of 200mph.

A boy is pulled from beneath a collapsed wall at the Plaza Towers Elementary School following a tornado in Moore, Okla., Monday, May 20, 2013. Photograph: Sue Ogrocki/AP

Authorities said that Plaza Towers elementary school in Moore took a direct hit from the tornado. The winds whipped off the roof, knocked down walls and turned the playground into a mass of twisted plastic and metal.

Rescue workers passed surviving children down a human chain to a triage center in the parking lot.

James Rushing, who lives across the street from the school, heard reports of the approaching tornado and ran to the school, where his five-year-old foster son, Aiden, attends classes. "About two minutes after I got there, the school started coming apart," he told the Associated Press.

A man with a megaphone stood near a Catholic church Monday evening and called out the names of surviving children. Parents waited nearby, hoping to hear their sons' and daughters' names.

Local reports said 75 children and staff were in the school at the time, some of whom were feared trapped in the rubble. KFOR reporter Lance West, who was on the scene, said he saw 30 children being pulled out alive.

Path of the tornado through Moore, Oklahoma.

Emergency services struggled to get to the badly hit areas because of damage to infrastructure and debris on roads. TV footage showed volunteers helping first responders search rubble for survivors in Moore. Sporadic fires broke out in the aftermath of the storm and thousands of properties were without power.

Oklahoma City police captain Dexter Nelson said downed power lines and open gas lines posed a risk in the aftermath of the tornado.

It was the second day that tornadoes had hit the area. On Sunday, tornadoes and baseball-sized hail brought destruction to a mobile home park in Shawnee, a suburb of Oklahoma City.

By Monday afternoon, two people were known to have died as a result of the severe weather at the weekend – 79-year-old Glen Irish and 76-year-old Billy Hutchinson.

Both were killed as tornadoes flattened homes in Shawnee. Governor Fallin declared an emergency in 16 counties as a result of severe weather and flooding.

A woman carries a child through a field near the collapsed Plaza Towers elementary school in Moore, Oklahoma. Photograph: Sue Ogrocki/AP

Residents had been urged to stay inside and take shelter before Monday's tornado.

Moore was struck by a severe tornado in 1999 that resulted in widespread destruction and resulted in the deaths of dozens of residents. On that occasion winds hit 302mph, registering F5 on the Fujita scale – the highest level.

Brian Edwards, meteorologist at Accuweather.com, told the Guardian that the storm was likely to spread out across a large part of the Great Plains and midwest into Monday evening. "We are in for a long night. It is a very, very large area covered and a very large storm. Some of this storm will hit as far north as Chicago and Milwaukee."

The National Weather Service said Monday afternoon that an area covering population of almost 62 million was at risk of storms.

• The Associated Press in Moore, Oklahoma, contributed to this report

http://www.guardian.co.uk/world/2013/may/20/oklahoma-tornadoes-moore-flattened