SECURITY for the Sydney and London marathons is under review after the explosions in Boston.

Athletics Australia said in light of the blast, which killed three at the finish line of the marathon, the organisers of Sydney marathon will look to beef-up security with NSW police to ensure the safety of the Sydney marathon in September.

Chief executive Dallas O'Brien said the association had ''deep concerns'' for all the Australians at the event.

He said the association had heard from its elite athletes competing in the event and all were unharmed.

But he said the terror attacks would mean security services will look closely at the Sydney Marathon - which runs over the iconic harbour bridge.

media_camera Marathon runner in tears after the two explosions at the Boston Marathon.

''We manage and run the Sydney Running Festival and we work really closely with all the authorities to put on that event. We run over the Sydney Harbour Bridge so all our risk management plans and security need to be really tight and spot on.

''We work really closely with NSW police and they are aware of what has happened and will be looking even more closely at the procedures for our event in September.''

Mr O'Brien said the Boston marathon was one of the most prestigious events in world athletics and attracted elite marathon runners.

He said the association had heard from Olympians Lee Troop and Jeff Hunt and also been assured paralympians Kurt Fearnley, Christie Dawes and Nathan Arkley were safe.

media_camera A Boston police officer clears Boylston Street following an explosion at the finish line of the Boston Marathon.

''It is an obvious tragedy and being an athletics event and having elite athletes involved our thoughts went straight to them. It has been great to hear that they are all well.

''They are very concerned and relieved that they are out of the trouble and holed up in their rooms. The leaders got through before the bombs went off, which maybe it was timed for that reason with the bigger numbers coming through.

''The place is a disaster zone at the moment and it is terrible.''

''We have deep concerns for all Australians involved and are willing to help any way we can, in taking phone calls from families or any other way.''

Meanwhile, security for the London Marathon this Sunday was already at a high but Metropolitan police chief superintendent Julia Pendry said it would be reviewed in the wake of the deadly American blasts.

''A security plan is in place for the London Marathon and we will be reviewing our security arrangements in partnership with London Marathon,'' Supt Pendry said in a statement.

The London marathon in its 33rd year is the world's largest and attracts more than 37,000 runners and crowds of up to 500,000 each year as it snakes its way through the British capital's most famous landmarks, finishing outside the gates of Buckingham Palace.

The London race's chief executive Nick Bitel said the Boston blasts were a sad day for athletes and ''our friends'' in marathon running.

"Our immediate thoughts are with the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running," Mr Bitel said.

''Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news,'' Mr Bitel said.

Mr Bitel continued: ''We will not be cancelling, what we are doing we're reviewing. You look at what has occurred, if there are steps we can take to increase security and all sorts of measures one could deploy. We run through the city, when you have an event of this nature, a marathon, a parade, it's only as safe as the city itself, if it's not held in a stadium you can't do a lockdown like you may do in a building.''

For operational reasons the security arrangements were not discussed.

London Mayor Boris Johnson Tweeted: ''Shocked by events at the Boston Marathon - my thoughts and prayers are with the victims and their families tonight.''

The British Foreign Office has been frantically attempting to confirm reports there may have been Britons injured in the blast.

Security about central London has been high for the past 48 hours in preparation for the funeral of former Prime Minister Margaret Thatcher.

Yesterday, police were checking and sealing manhole and letterboxes and looking at security on a route from Westminster to St Paul's Cathedral.

Amid general fears of a terrorist attack during the major event to be attended by the Queen, Prime Minister David Cameron and his Cabinet and leaders and dignitaries from around the world heightened security has meant specialist police and military counter terrorism units had been rostered for London duty this week.

Security arrangements are not being changed for the Thatcher funeral.

But those same units on both land and on the Thames River may now have a role for the London Marathon.

British Prime Minister David Cameron and Foreign Secretary William Hague offered their condolences to those affected in Boston and were being updated on security for London's events.

The Opposition's spokesman for treasury and defence Ed Balls and Jim Murphy respectively are both due to run the London Marathon.

Mr Balls Tweeted: ''awful and tragic news from Boston - nothing more uplifting to the spirit than a cheering marathon crowd - terrible to see this happen.''

Mr Murphy wrote: ''Too early to speculate who did it but need to look again at security for London this Sunday.''

There are no credible or specific threats against the London Marathon but police, off the record, confirmed security would be increased.

The terror threat in London generally remains as ''substantial''.

There are 374 registered British runners in the Boston Marathon who the Foreign Office are currently trying to trace.

A series of suicide bomb attacks on London Transport in 2005 killed 52 people and injured hundreds more prompting the government's long term ''substantial'' threat alert for the capital.

-with Lanai Scarr

Originally published as Sydney Marathon security under review