The male cyclist was taken to hospital with a serious head injury after the crash with a white Mercedes Sprinter van at around 10.20am on Mondayon Temple Bar.
The cyclist remains in hospital in a critical but stable condition, police said.
The driver of the van, a man aged 32, was arrested on suspicion of driving a motor vehicle while unfit through drugs. He has been released pending investigation.
Detective Sergeant Paul Hughes, from the serious collision investigation unit, said: “I’m appealing to anyone who was in the area at the time and may have seen what happened to contact me.
“If you have dash cam footage or any information which can assist our investigation I’m keen to hear from you.
“We’ve got specialist family liaison officers supporting the elderly man’s family during what is a worrying and difficult time for them.”
Anyone with information is asked to contact police via Live Chat on the website, by calling 101 or emailing FL_COLLISION_INVEST@west-midlands.pnn.police.uk, quote log 1267 of 15 June.
Alternatively, contact Crimestoppers anonymously on 0800 555111 or at crimestoppers-uk.org