Emergency services are at the scene of an explosion at a block of flats in east London.
Ten fire engines are at the Bridge Point building in Hornchurch, along with the Metropolitan Police and the London Ambulance Service (LAS).
Scotland Yard said it was called to the scene just before 5.15pm to reports of an explosion at the flats.
An LAS spokeswoman said two people had suffered minor injuries.
Scotland Yard said officers were unable to confirm the cause of the explosion.
Residents were being evacuated from the building and road closures were in place.
Transport for London said the A127 Southend Arterial Road was shut between Ardleigh Green Road and Bryant Avenue.
Police later said two people were taken by LAS to an east London hospital, two were being treated at the scene, and one went to hospital themselves.
Jason Smedley, 46, managing director of Nail Harmony UK, close to the Bridge Point building, said he was in his office at the time.
"We felt the whole building shake," he told the Press Association, adding that he could soon see "flames billowing".
Mr Smedley said the front of the flats had been "blown out" across the road.
"We pulled people back and told them to get out of the way," he said, adding that he and his staff brought people back to their office to give them shelter.
"Unfortunately there was a woman and her daughter, that were in the flat above, that heard the explosion, ran out, and we brought them back ... had just their dressing gowns on, so we just gave them tea and blankets. They've got nowhere to live tonight," he said.
Mr Smedley wrote on Facebook: "Carnage!! Never witnessed anything like that before. We all heard a massive explosion.. ran round to see the front of the house across the a127! Pray that no one was in the flats. Stomach churning!!"
A spokesman for London Fire Brigade (LFB) said the incident is still ongoing.
LAS said it has treated four patients at the scene and taken them all to hospital.