LONDON (CBS/AP) - Armed police have closed a section of the M6 motorway in both directions following a police-led incident at the Weeford toll plaza in central England.
Police said in a statement Thursday that the incident in the West Midlands, around 116 miles north of London, is ongoing.
Armed officers surrounded a bus which was traveling to London from the country's north on the major highway.
Witnesses told British television networks that the commercial bus stopped of its own accord, pulling over after an allegation against one of the passengers, and the police arrived on the scene afterwards.
One eyewitness reportedly said a man was seen to pouring a liquid into a bag, or a box, that was smoking. One-by-one, the passengers were asked to leave the coach and were led away by police.
U.K. television showed images of hazardous materials response teams from the emergency services setting up a decontamination tent on the motorway, which is one of England's primary north-south routes.
Sources tell CBS News it was not a planned counterterrorism operation, and was unrelated to the arrest of six people in a counterterrorism operation earlier Thursday morning.
Officials say five men and one woman were arrested on suspicion of terrorism in what the Metropolitan police have called a "significant" operation, though they stress that these arrests were not linked to the pending Olympic Games. One man, however was arrested in Stratford, where the Olympic Games are being held.
Armed British police carried out the early-morning raid on a residential address near the Olympic Park, using smoke grenades and a stun gun in what they said was a pre-planned anti-terror operation.
Police didn't identify the suspects, who range in age from 18 to 30. However Mizanur Rahman, who is involved with a Muslim activist group, said all those arrested were British Muslims. He said the three men arrested in Stratford, the neighborhood that contains Olympic Park, were brothers and claimed that one was a former police officer.
Resident John Smallshaw said one of the raids took place at a residence on Abbey Road, only about a mile from the Olympic Park.
He told The Associated Press he was woken up just after 4 a.m. by "five loud bangs in quick succession" and saw police raiding the house. He said he later witnessed "one young man taken on foot to a waiting ambulance."
Another man was later taken into an unmarked police car, he said, adding that plainclothes officers were still at the premise.
Continuing searches are being carried out at eight separate addresses in east, west and north London and at one business in east London, police said. Rahman identified one of the addresses as being in Old Street, near London's financial district.
Police said all the suspects have been taken to a southeast London police station.
Intelligence officials say there has been an expected increase in chatter among extremist groups, but there are still no specific or credible threats targeting the Olympics. The terror level is labeled substantial, a notch below severe. A substantial threat level indicates that an attack is a strong possibility