Police investigating a potential terrorist attack said they had arrested a seventh person, a 22-year-old woman, in east London on Saturday.
Police are on high alert ahead of the London Olympics but said the latest arrest and those of a woman and five men in London earlier this week were not linked to the Games.
All seven suspects have been held on "suspicion of the commission, preparation or instigation of acts of terrorism", police said.
Britain has spent millions of pounds beefing up security in preparation for the Olympics.
Security chiefs have said repeatedly that they have no information that the Olympics are being targeted, but Jonathan Evans, head of the domestic intelligence agency MI5, has said the Games present an attractive target.
In a separate operation this week police arrested seven men on suspicion of terrorism after weapons were found in a vehicle stopped on a motorway in Yorkshire, northern England.
A police source said that in that case too, there was nothing to suggest any link with the Olympics, which start on July 27.
In both cases security sources have said the suspects were linked to militant Islamism, but that it remained unclear what was planned. The London suspects were arrested when their plotting was at an early stage, the sources added.
In a sign of heightened vigilance ahead of the Games, armed police closed the M6 motorway near Birmingham, in the Midlands, for four hours on Thursday after a man was reported acting suspiciously on a coach heading to London.
It later emerged the alert was caused by a passenger using an electronic cigarette.
Security authorities have assessed the national threat level at "substantial" - meaning that an attack is a strong possibility - but that is one level lower than it has been for most of the time since the July 7, 2005 suicide bomb attacks in London which killed 52 people.