Part of Manchester's busy city centre was evacuated, with bomb squad officers called after a "suspicious item" was found on a bus.
Piccadilly Gardens was sealed off and witnesses said they have been told to leave the area as "a matter of urgency", while a local councillor said there may have been three incidents in other locations.
Greater Manchester Police officers were called to the scene just after 1pm after the item was discovered.
Three hours later, specialist officers gave the all-clear and the cordon started to be lifted.
The bus was evacuated as a precaution and a cordon was set up around the scene, with several roads closed and pedestrians asked to move back.
Councillor Pat Karney, who posted photos and video of the police activity, said what looked like a "bomb squad" was on site.
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He tweeted: "Crowds been pushed back from Piccadilly presume for our safety.
"Hearing bus people say there have been three incidents today in other locations."
He also wrote: "Looks like the bomb squad here. Its edging towards the green bus in the station."
Mr Karney said a helicopter was flying above the city centre and four ambulances were on scene, in addition to several police vehicles.
A witness wrote on Twitter that "lots of police" were "telling people to leave area as 'matter of urgency'".
One woman who was caught up in the drama said she was at a Slug & Lettuce bar, which was evacuated.
She wrote on Twitter: "Currently trapped in Slug & Lettuce at Piccadilly Gardens because of the police cordon."
She later tweeted: "Just been evacuated from Slug & Lettuce."
Bus and tram services were disrupted.
A police spokesperson said: "At around 1.15pm today, police were called to reports that a suspicious item had been found on a bus at Manchester Piccadilly bus station.
"Officers are in attendance and several roads are closed.
"The bus has been evacuated as a precaution and a cordon remains in place.
"Enquiries are ongoing."
The force added: "Please avoid the Piccadilly Gardens/Piccadilly Bus Station area, whilst we deal with an ongoing incident."
