The police were called after an allegation was made against one of the 48 passengers on board the coach travelling from Preston to London.
A spokesman for Megabus said: “We are assisting police with their inquiries into an allegation made against a passenger who was travelling on board one of our services.”
Armed police officers were positioned next to the bus as passengers were led off the vehicle one by one.
Reports suggested that a man was seen pouring liquid into a smoking bag on board.
The bus is understood to have pulled over on to the hard shoulder. The Highways Agency said that the road is closed in both directions between junctions three and four.
The incident is not thought to be linked to the anti-terror raids staged in London this morning, in which six people were arrested over an alleged plot to attack targets in Britain.