The Metropolitan Police said a judge agreed late on Wednesday to give detectives more time to grill the suspects, British citizens aged 17 to 32, under the Terrorism Act 2000.
Under the law, the police could apply to keep the suspects for a maximum two weeks.
They were taken into custody on Tuesday when 700 police officers in raids on 24 locations in and around London which also netted a half-tonne of ammonium nitrate fertiliser, a compound that could be used to make explosives.
Police have said there is no connection between Tuesday's operation and the Madrid train bombings on 11 March which killed over 200 people and renewed fears of attacks on the British capital.
But Spain's outgoing interior minister Angel Acebes said that one of the suspects might have had a connection with the Madrid bombings.
Blair praises Muslims
British Prime Minister Tony Blair was expected to comment on this week's raids during his monthly Downing Street press conference on Thursday.
In parliament on Wednesday, Blair applauded an appeal to the nation's 1000-plus mosques from the Muslim Council of Britain to condemn terrorism and support police in their investigations.
The appeal was criticised in other Muslim quarters as pandering to rampant Islamophobia.
Police arrested the eight suspects in raids on homes and businesses in London and the surrounding counties.
Their names have not been disclosed by police, but at least three are related to each other, according to family members.