London terror plots: Six held, bus emptied on highway

London - Armed police and forensic officers with sniffer dogs Thursday swooped down on a bus moving on a highway near London and asked all passengers to get down after a man was seen pouring a liquid into a box, which then started emitting smoke, BBC reported.

Earlier in the day, six people, one of them a woman, were arrested in London for their alleged involvement in terrorism offences.

The report said the arrests are related to a possible plot involving Islamist extremists, with potential British targets. However, they were not linked to the Olympics or Paralympics, police said.

The passenger coach - on its way from Preston to London - was stopped on the M6 toll motorway in the West Midlands, and the motorway closed in both directions near Lichfield.

The 48 passengers were led off the coach and forced to sit apart in a cordon on the road while surrounded by officers.

Police said the coach stopped "of its own accord".

Officers in forensic suits and others in military fatigues checked the area while fire service and ambulance crews were also involved in the operation.

The ministry of defence confirmed military personnel were assisting police.

The passengers were later moved to another coach which remained parked under the canopy of the nearby toll booth.

A car driver named Nick Jones, from Cambridge, said he was about four vehicles behind the coach when police stopped traffic.

"We were told to stay in our cars, keep windows up and not put the air conditioning on," he said.

Earlier in the day, six people, one of them a woman, suspected to be involved in terrorism offences were arrested in London in a "significant" operation led by British intelligence agency MI5, BBC reported.

Some of those arrested on suspicion of preparation or instigation of acts of terrorism are British nationals.

They include a 21-year-old man and a 30-year-old woman, who were detained at separate addresses in Ealing, west London.

A 29-year-old man was also arrested in Ealing, while three other men, aged 18, 24 and 26, were detained in Newham, east London.

Security sources described the arrests as "significant".

Britain's terror threat level is still "substantial", which means a terrorist attack is a "strong possibility".

There are two higher levels - "severe", meaning an attack is "highly likely", and "critical", meaning an attack is "expected imminently", BBC said.