There were several blasts across London
Networks in London were running at near capacity as those caught up in the chaos tried to call family and friends.
The surge in the number of calls being made meant that many people had trouble getting through first time.
To limit congestion, network operators urged those using their mobiles to keep calls as short as possible following the explosions across London.
Domino effect
There were three explosions over 30 minutes during the rush hour on trains on the Underground network. Another blast ripped the roof off a bus in the centre of the city.
More than 30 people were killed in the explosions on the Underground.
We're asking that people only make essential calls to limit the congestion
BT spokesman
Blogs respond to blasts
Network operators said they had boosted capacity to handle the surge in calls, but asked that people refrain from making unnecessary and lengthy phone calls.
O2 said it had doubled its network capacity, while Vodafone said it had also increased capacity and had freed channels for use by emergency services.
Shortly after the explosions, a spokeswoman for Virgin Mobile, which piggybacks on the T-Mobile network, said: "There are so many people making calls at the moment it is taking a while for people to get through. The volume of calls has really surged."
Many of those caught up in the chaos who found that the mobile networks were down reportedly went into shops to beg the use of a phone.
Essential calls
Terrorism expert Professor Michael Clarke from the International Policy Institute at King's College London, speculated that the problems might be a security measure.
"I've heard rumours that the mobile network is down, possibly shutdown," he said.
"This could be because the MO (modus operandi) in Madrid was by setting off devices with mobile phones."
But mobile firms denied that the government had used emergency powers to shut down the networks.
A spokesman for Orange said it had not been asked to suspend its network, and said any problems people were experiencing were due to call volumes.
The problems with the mobile networks has had a knock-on effect on fixed lines phones.
A spokesman for BT said that so many people were turning to fixed line phones once they found that the mobile network was down that it was running near capacity too.
"Due to the sheer volume of calls, the network is experiencing some congestion problems," he said.
"We're asking that people only make essential calls to limit the congestion," said the spokesman.
Amid the confusion surrounding today's London blasts, officials have urged the public to avoid the city if possible to allow emergency services to operate more quickly.

All rail and underground stations remain closed, although efforts are being made to reopen them. Air links are now closed at Stansted, Heathrow, Gatwick and London City airports.

Those trying to contact friends and family via mobile have struggled as the largest mobile networks, Vodafone, O2 and Orange, have all suffered from full capacity on several occasions this morning meaning voice calls failed. 

On the web, websites such as BBC, TfL and the  G8 2005 website struggled for a short time at one point this morning under the strain of so many web users visiting them. The national grid has said that it has sustained no damage to the electricity network.
On the  Traffic England site run by the Highways Agency, London has been marked as a 'Closed area'.

Euston Station cordoned off

The Royal Mail is warning customers that mail collections and deliveries across the UK will be disrupted because of the incidents.  Royal Mail vehicles will not be moving in or out of London, or between Royal Mail sites in central London for the remainder of today.  At least 25 per cent of the UK's mail moves through London, even if its end destination is elsewhere.  Disruption may therefore be felt throughout the UK.  Royal Mail expects to issue further information later today.

Bus services in Zone 1 have resumed as of  16:00 BST.