A 22-year-old woman has been arrested in east London in connection with an ongoing counter-terrorism operation, the Metropolitan Police has said.
Officers arrested the woman at a property in Hackney during the early hours of Saturday morning, it added.
She is being held on suspicion of the commission, preparation or instigation of acts of terrorism.
Police have been given until 12 July to detain six people arrested on Thursday as part of the same investigation.
Detectives from the Met police's counter-terrorism command were granted additional time for questioning by a magistrate on Friday.
The six, five men and a woman held on Thursday, are also being questioned on suspicion of the same offence as the 22-year-old woman.
One of the six, who are aged between 18 and 30, was Tasered by police.
Armed officers were involved in the arrests at the property in Stratford
It is understood the arrests relate to a possible plot involving Islamist extremists, with potential UK targets.
Three of those held are brothers - one an ex-police community support officer.
He served with the Metropolitan Police for more than two years before resigning in September 2009.
The force said he had not been "deployed in any specialist or sensitive roles".
The brothers, aged 18, 24 and 26, were detained in Abbey Road, in Stratford, east London, during an operation involving armed officers.
The BBC understands they are Jahangir, Mohammed and Moybur Alom.
Richard Dart, a Muslim convert who uses the name Salahuddin al Britani, was also detained in the raids, it is understood.
He features in a YouTube video which criticises the Royal Family and British military action in Muslim countries.
The Metropolitan Police said the operation is not connected with the Olympics or Paralympics.