Though the London police refused to be drawn into discussing the aborted plot, both British and American media on Tuesday claimed security services had foiled it after intercepting communications between potential attackers.
The plan was apparently to detonate a combination of explosives and a highly toxic substance called osmium tetroxide.
Media reports said the suspected plotters were not thought to have succeeded in obtaining any of the chemical.
They said the potential target was an area in which there could be concentrations of people, possibly within a confined space.
"The plotters were thought to be sympathetic to the aims of al-Qaida and the intended target was believed to be British civilians," the BBC said.
Osmium tetroxide, which is used mainly in research laboratories, can attack soft human tissue and could blind anyone who breathed in its fumes or cause them to choke to death.
Security services are believed to have been alerted after a mention of osmium tetroxide was picked up by a British government electronic listening centre in Cheltenham, southwest England.
Nine people were held in and around London. A half-ton of ammonium nitrate fertiliser, which can be used to make explosives, was seized.
The UK, having invaded Iraq with the US, has been on heightened guard against possible "terror attacks".