Chris Denning is the 13th person to have been arrested under Operation Yewtree - the investigation sparked by the Jimmy Savile scandal.
The investigation is being run in three strands and the allegations against Denning fall into the category that does not involve Savile.
Mr Denning was part of the original DJ line-up when the BBC launched Radio One in 1967. He had previously worked as a BBC2 announcer.
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Two knives were found at the scene in Islington, North London, and the suspects fled on foot before being arrested.
The violence began after a counter-protester allegedly doused a female rally organiser with pepper spray.
As temperatures climb through the day, the warmest spots are likely to be across western parts of Northern Ireland and western Scotland.