Two men have been arrested after an explosion at a block of flats.
Emergency services were called to the Bridge Point building in Hornchurch, east London, just before 5.15pm on Monday where 10 fire engines tackled the resulting blaze.
The Metropolitan Police said two men had been arrested on suspicion of arson, adding that both were taken to hospital for treatment.
Police were unable to confirm the cause of the blast at this stage.
London Ambulance Service (LAS) said four patients were treated at the scene and taken to hospital, one of whom was taken as a priority.
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 Havering Council spokesman said most residents went to a local McDonald's restaurant, adding that measures will be put in place to ensure they had somewhere to stay.
The council spokesman said the explosion came from a ground floor corner flat, adding that 41 flats had been evacuated, with many residents staying with family and friends.
Police said road closures were in place and people were advised to avoid the area.