Three people - including a police officer - have been stabbed by a man wielding a knife at Manchester Victoria railway station.
A man and woman in their 50s were attacked at about 20:50 GMT, Greater Manchester Police said.
A British Transport Police officer was stabbed in the shoulder. The victims' injuries were described as "serious" but not life-threatening.
A man has been held on suspicion of attempted murder.
Officers said counter terrorism police were leading the inquiry, but they were keeping an open mind on the motive.
The officer, a police sergeant in his 30s, was released from hospital on Tuesday.
BBC 5 live producer Sam Clack, who had been at the station at the time, said he saw a man stabbed on a tram platform at the station, which is next to Manchester Arena, "feet from me".
Mr Clack also said he heard the knifeman shouting "Allah" during the attack, along with a slogan criticising Western governments.
He added he was "close to jumping on the tracks" as the attacker had a "long kitchen knife".
Greater Manchester Police, which declared the attack a "critical incident", said it understood that people would be worried but there was "no intelligence to suggest that there is any wider threat at this time".
Mr Clack said he first "heard this most blood-curdling scream and looked down the platform".
"It looked like they were having a fight, but the woman was screaming in this blood-curdling way. I saw police in high-vis come towards him. He came towards me.
"I looked down and saw he had a kitchen knife with a black handle with a good, 12in blade. It was just fear, pure fear."
He said police used pepper spray and a Taser on the detained man, who he said had been "resisting arrest".
He said he saw "six or seven" officers on top of the man, who he described as "very skittish".
"He was very aggressive and very intent on causing more harm than he actually did," he said.
"It was very, very scary."
Police said the woman had injuries to her face and abdomen and the man had injuries to his abdomen.
The New Year firework display in Albert Square went ahead.
City councillor Pat Karney said the authority was being kept informed by Greater Manchester Police.
Just come in from Albert Sq.Our thoughts are with the people injured tonight.The City Council is being kept informed by GMP.— Pat Karney (@patkarney) January 1, 2019
Assistant Chief Constable Rob Potts said: "Tonight's events will have understandably worried people but I need to stress that the incident is not ongoing, a man is in custody and there is currently no intelligence to suggest that there is any wider threat at this time.
"While we don't yet know the full circumstances and are keeping an open mind, officers from Counter Terrorism Policing North West are leading on the investigation."
You may also like:
Manchester Metrolink said no trams would be calling at the station "at this time".
Due to a Police incident at Victoria Station we are unable to operate any trams through at this time. We apologise for this, please check the website for updates https://t.co/IaCKtpmVEE— Manchester Metrolink (@MCRMetrolink) December 31, 2018
National Rail Enquiries said there would be no arrivals or departures at Victoria station for "the next two hours".
Manchester Cathedral, which is a five-minute walk away from the station, also reacted to what had happened.
Thoughts and prayers are with those injured during the incident at #manchestervictoria.— Manchester Cathedral (@ManCathedral) December 31, 2018