"We are piecing together what took place and speaking to witnesses to understand how this incident unfolded.'' said Detective Senior Sergeant Brian Archer.
"It is early days yet and we have some work to do to establish exactly what happened.''
Police appealed for any other witnesses to come forward.
Police were also reviewing CCTV footage from the area.
New Zealand Cricket chief executive David White and other officials have flown to Christchurch to ascertain what had happened.
"We are all shocked by what has occurred and extremely concerned for Jesse. New Zealand Cricket's thoughts are with him and his family,'' he said.
Witnesses to the beating outside Aikmans Bar in Merivale reportedly said Ryder was attacked by four men, with one describing it as an "unprovoked'' attack.
A witness, who only wanted to be known as Adam, told Stuff.co.nz that Ryder had his shirt ripped off his back and was on the ground as several men kicked and punched him outside the bar.
Ryder was lying in a bush "shaking, vomiting and covered in blood'' in the carpark of McDonald's, which was across the road from the bar.
Merivale McDonald's owner Murray Traill said he was investigating the incident and going through security footage.
The restaurant was closed at the time and only the drive-through was operating.
Adam, who was drinking at the bar, told Stuff the brawl broke out behind him.
"From what I could tell it looked pretty unprovoked,'' he said. "Four dudes were just laying into him and absolutely smashing him on the ground. His shirt was ripped off and they were kicking him and punching him while he was down.''
Police confirmed they were investigating an incident involving Ryder at a Merivale bar early today.
St John Ambulance said two ambulances were sent to an alleged assault on Papanui Road at 12.34am.
"We transported to Christchurch Hospital ED under lights and sirens one patient, a male, aged in his 20s, with very serious head injuries.''.
Aikmans Bar general manager Steve Holmes confirmed Ryder had been at the bar with Wellington Firebirds teammates last night.
He said the incident started outside the bar.
He had reviewed security camera footage and had given it to police but would not say what it showed.
Nor would he say if Ryder had been drinking alcohol in the bar.
He did reveal that the mother of one of the men who attacked Ryder was there when it happened.
Mr Holmes said the attackers tried to get back into the bar after the assault but were blocked by bar staff.
The mother of one of the men then began arguing for them to be allowed back in before they all left.
Mr Holmes wished Ryder all the best and hoped police would quickly track down his attackers.
It was a typical, quiet Wednesday night and no bouncers were on the door.
Blood could be seen on the footpath directly outside the entrance to the bar and a long trail led to a Mcdonald's carpark.
Speaking at nearby St Andrews College today, Prime Minister John Key talked about the incident, Newstalk ZB reported.
"It sort of looked as if it was somewhat unprovoked, so there must be something behind that and the assault seems extremely vicious. Obviously we wish him a speedy recovery. I'm sure the police will be working to try and get to the bottom of it,'' he said.
Wellington Firebirds players were in a sombre mood at Christchurch International Airport as they flew home this morning.
Blackcap and senior squad member Grant Elliott said they were all waiting for some good news.
"It's not great, is it?'' he said.
"But we don't know how serious it is. We're not too sure what's happening - it's all pretty confidential at the hospital.
"We're all just hoping for the best.''
Elliott would not say who was out with Ryder or comment any further.
Ryder's family and cricket officials are flying to Christchurch to support him this morning.
They include White, Peter Clinton from Cricket Wellington, Heath Mills from the New Zealand Cricket Players' Association and Ryder's manager Aaron Klee.
Ally Green was drinking at Aikmans Bar from 6pm to 11.30pm yesterday.
She said a group of six or seven young guys were in the bar wearing Wellington colours.
They were all drinking but they were quiet, and did not seem drunk.
"Everyone was in a good mood,'' Ms Green said.
"No one seemed out of control. It was that group and us, the rest were all older.''
She left in a taxi about 11.30pm and there was nothing out of the ordinary.
"I kinda wish I stayed now.''
Klee wrote on Twitter: "Thanks for the calls and msgs of support for Jesse. Just heading to Chch to see him.''
Crusaders player Zac Guildford tweeted: "Thoughts and prayers with @DijaRyder much love''.
Monty Betham, whose gym Ryder trained in, tweeted: "Come on @DijaRyder show the fight you have within thinking of you brother.''
Ryder was in Christchurch playing for Wellington in the major preliminary final against Canterbury at Hagley Oval yesterday, which Wellington lost by 61 runs. Ryder was dismissed for a golden duck during Wellington's unsuccessful run chase.
He had been due to depart New Zealand this week to join the Delhi Daredevils for this year's edition of the Indian Premier League where he was bought by the franchise for $USD260,000 (NZD310,577) in February's player auction.
In February 2008, Ryder cut his hand trying to break into a toilet at a Christchurch bar at 5.30am the day after New Zealand won a One Day International series against England.
The following January, he failed to turn up for team training after getting drunk in Wellington following an ODI against the West Indies in Wellington.
Later that year, Ryder whacked a chair with his bat after being dismissed and was fined 15 per cent of his match fee. When he was chastised by team manager Dave Currie, Ryder reacted with a tirade of abuse.
In July 2010, Ryder was charged with serious misconduct over being drunk and disorderly during an indoor cricket competition in Christchurch.
He was then axed from the third one-day international in Auckland in March last year after he became involved in a verbal altercation with patrons at a Napier bar.
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- nzherald.co.nz with APNZ and Newstalk ZB
