An unexploded World War Two bomb discovered in north-west London could take three days to safely detonate, police said.
Emergency services descended on a building site in Brondesbury Park on Thursday after construction workers found the huge bomb.
Hundreds of pupils from Marylebone Boys’ School and Malorees junior and infants schools were evacuated as well as nearby residents.
Police officers erected a cordon around the site, on the Avenue, while army bomb disposal experts assessed the situation.
Scotland Yard confirmed the operation with continue until at least Friday.
Councillor Carol Shaw, for Brondesbury Park ward told the Standard: “The construction firm have been told that it might not be clear for three days,” she told the Standard.
“The police said a specific part from Gloucestershire may be needed to safely detonate the bomb.”
Temporary shelters have been set up for those evacuated from their homes by police officers.
“It’s very major and it could be very dangerous, which is worrying,” added Cllr Shaw.
The Met Police could not confirm a time frame for the safe disposal of the bomb, but admitted it would be a major operation.
Two fire engines and 14 firefighters were also sent to the site, near Brondesbury Park station, after emergency services were called at 11.30am.
Property developer Regal Homes is currently constructing 74 apartments on the 0.8-hectare site, with work beginning last year.
A spokesman for the firm said: “Excavations on site at The Avenue, Queen’s Park, have unearthed an object of concern. The health and safety of our team and our neighbours is of course our primary consideration and Regal Homes is cooperating fully with the authorities who are investigating the situation.”
Roxanne Landripet, who lives opposite the building site, also said there were fears the road might remain cordoned off for the next two days.
She described the “panicky” scenes as people were evacuated from their homes.
"There were police banging on people's doors and then I started to hear a helicopter flying around above the road," said the 24-year-old receptionist.
"A police officer knocked on my door and told me we had to leave because an unexploded World War Two bomb had been found. There were fire engines, ambulances and police cars everywhere."
Cllr Muhammed Butt, Leader of Brent Council, said: “Council officers are currently making arrangements for our residents in the Willesden Lane area to make their way to a rest centre where they will be looked after until it is safe to return to their homes.
“I would like to thank these residents who have shown great patience during this time. We will continue to provide updates on the situation as soon as they come through.”
Superintendent Nigel Quantrell of Brent Borough, said: "I would like to thank the community for their continued support as we and our partners deal with this incident. I can assure you that we are working very hard resolve the situation. The public's understanding and cooperation is deeply appreciated."
A reception area in Mortimer Road has been opened by the London Borough of Brent to assist those residents affected. They will be laying on local transport from the area.
If you want to check on an affected resident who lives in the area call the local authority helpline which is 020 8937 5788.