Four suspected Islamist terrorists have been arrested in a series of raids across London.
One of the suspects - aged 21 - was Tasered but was not injured and did not require medical treatment, Scotland Yard said.
Counter-terror police arrested the men – two of the others aged 20 and a third aged 21 – and were helped at one address by armed officers, but no shots were fired.
It is understood at least one of the suspects is believed to have travelled to Syria.
A Whitehall source said officers believed the raids were an "early disruption" of what could have been a significant plot.
A Scotland Yard statement read: "These arrests and searches are part of an ongoing investigation into Islamist-related terrorism."
All men were arrested on suspicion of being involved in the commission, preparation or instigation of acts of terrorism.
Metropolitan Police Commissioner Sir Bernard Hogan-Howe described the raids as "a quite serious case".