Police hunting James Allen, a man suspected of the murders of two people in Middlesborough, announced on Sunday morning they had arrested a 36 year-old man in south Leeds.
Allen was wanted over the killings of Julie Davison, 50, in Whitby, North Yorkshire, and 81-year-old Colin Dunford in Middlesbrough.
The arrest came as a third police force joined the manhunt for a violent convicted criminal suspected of killing two people in their own homes following a fresh sighting.
Speaking at a news conference at Cleveland Police headquarters, in Middlesbrough, temporary Detective Chief Superintendent Gordon Lang praised the officer who spotted Allen for an "excellent piece of police work".
Describing this morning's arrest, Mr Lang said: "Just after seven o'clock this morning in Leeds an off-duty police officer saw a man. His suspicions were raised around this man, he took up observations, maintained observations, called for assistance. That assistance arrived and, after a short foot chase, that man was arrested."
He added: "I have to say what an excellent piece of police work by an off-duty police officer."
An urgent appeal to find Allen was issued by Cleveland Police and North Yorkshire Police following the murders of Julie Davison, 50, in Whitby, North Yorkshire, and 81-year-old Colin Dunford in Middlesbrough.
And West Yorkshire Police joined the search after Allen was seen on CCTV footage walking along Harehills Lane at 2.41pm on Friday.
Police also released CCTV images of Allen in Scarborough earlier this week.
The pictures showed Allen in a cash exchange in the seaside town on Tuesday and Wednesday and in a sports shop on Wednesday.
Mr Dunford, who was last seen alive at around 5pm last Sunday, was found dead with serious head injuries at his terraced house in Leven Street, Middlesbrough, on Monday after he failed to appear at Samuelsons Working Man's Club, where he was a daily visitor.
Ms Davison, who lived alone, was found dead by her sister at her rented flat in Church Square on Wednesday. She had also suffered serious head injuries, police said.
Police described the killings as "two brutal murders in the victims' own homes".Yesterday, police also released CCTV images taken of Allen in Scarborough during the week.
The pictures showed Allen in a cash exchange in the seaside town on Tuesday and Wednesday and in a sports shop on Wednesday.