British citizens in the Boston area are being urged by the Foreign Office to "Let family and friends know you are safe".
People who are in the city or are planning to travel there imminently are being told to "exercise vigilance and caution, monitoring local media and following the advice of local authorities".
The Foreign Office also refers travellers to the Massachusetts Emergency Management Agency, which is warning people to avoid the Copley Square area of Boston where the explosions took place - and to register at redcross.org/safeandwell.
About 2,000 people fly each day from London Heathrow airport to Boston, on six separate flights. All flights today, on Delta, Virgin Atlantic and British Airways, are going ahead as normal. BA, which is the main airline with four flights, says passengers booked to travel today, tomorrow or on Thursday can postpone their trip up to 29 April, or switch destination without penalty.
Security at Boston's Logan airport is expected to be extremely tight: this was the airport from which American flight 11 and United flight 175 departed on 11 September 2001, and which hit the north and south towers respectively at the World Trade Center in New York.
The city authorities have said that there will be random searches in the days ahead in a bid to avoid further atrocities.
In downtown Boston, transport is widely disrupted. The blasts took place on Boylston Street, one of the main thoroughfares in Back Bay - an upmarket area with some of the city's top hotels and restaurants, frequented by both tourists and business travellers. This is is one of the main hubs of the "T" subway system.
Boston is a key business destination, on a par with Chicago, San Francisco and Los Angeles. UK firms and travel management companies are identifying who is in the Boston area and who is booked to travel there - and making decisions about whether to go ahead with travel plans or bring back business travellers. For holidaymakers it's a different picture. For anyone with what's called "Disinclination to travel", there is currently no option to cancel plans without penalty - though that may change as the airlines get to work and assess the picture in Boston.
One of Boston's "home town" airlines, JetBlue, was a sponsor of the Boston Marathon. It has said anyone booked to travel to or from Boston today or tomorrow can change their plans free of charge.
There was also a fire at the JFK Library - a memorial to President Kennedy, who came from Boston. It is both an academic research centre and a tourist attraction on the waterside south of the city. It appears that the fire may have been a mechanical problem and purely coincidental.
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