Mobile phone networks are recovering after struggling in the hours immediately after the London blasts.
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.
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.
People in the capital, anxious to know the fate of friends, relatives and employees, found themselves unable to make or receive calls or send text messages.
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.
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.