Emergency services have advised people to start making their way out of London, following earlier explosions on the Underground and on a bus.
The Tube network has been suspended but a limited service may resume on Friday. Central London bus services started to operate again on Thursday afternoon.
People are advised to try to stagger their journeys to help the system cope, and to be prepared for long delays.
The police have set up a casualty bureau number on 0870 1566344.
Callers who cannot get through straight away are advised to be patient, as demand is likely to be high.
The Met Police has urged people who have managed to get home to let friends and relatives know. This will help relieve pressure on the emergency number.
Officers would also like people who were injured or involved in events to call the casualty bureau to say they are safe.
Mainline rail stations have been re-opened to help people get home
The Met earlier said it would continue to respond to 999 emergency calls but non-emergency calls would have a seriously delayed response.
Network Rail said all mainline stations except King's Cross are now open. It is hoped some suburban services may operate from King's Cross later.
Passengers are advised to check train operators' websites for updates to services.
Docklands Light Railway has resumed but will not be stopping at Bank, Stratford and Canning Town.
Tim O'Toole, of London Underground, said it was hoped unaffected parts of the Tube system would re-open late on Thursday and that a limited service would run on Friday.
He said the Jubilee, Northern and Victoria lines were unaffected and that parts of the Circle and Piccadilly lines might also re-open. The Hammersmith and City and Metropolitan lines are likely to remain closed.
Transport for London (TFL) said central London bus services were slowly returning to normal on Thursday afternoon. Buses are being searched regularly.
Westminster City Council said pupils should remain inside school buildings and carry on normal activities.
Local authorities and police said they would ensure children were looked after safely until parents could come to collect them.
The Met has urged anyone who does not need to come into London on Thursday not to do so. People already in London are encouraged to limit travelling as far as possible.
Airports are operating normally but with some delays to flights.
The Federation of Tour Operators (FTO) advised passengers due to fly from London airports still to go to check in, even if disruption to public transport meant they arrived late.
London's congestion charge has been suspended for the day.
Medical treatment has been co-ordinated by University College Hospital, with casualties also treated at the Royal London Hospital, St Mary's Hospital, the Royal Free Hampstead NHS Trust and St Thomas' Hospital.
People have been asked to make only essential mobile phone calls, as networks are operating at near capacity.