Izzet Eren was jailed for 21 years in 2015 for firearms offences and while serving his sentence he was transferred to a prison in Turkey in August 2019.
The Met said Eren absconded from that prison a month later.
Commander Fiona Mallen said extradition proceedings were under way after Eren’s arrest in Chișinău on Thursday (May 26).
She said: “The Metropolitan Police Serious Crime Manhunt team works around the clock to track down the criminals ‘most wanted’ by the Met.
“In this case, a hugely complex investigation was launched to establish Eren’s whereabouts, with a wide range of investigative and sensitive intelligence opportunities exploited.”
Eren was caught in October 2015 carrying a loaded pistol and a machine gun in north London while allegedly on his way to carry out a shooting.
While being transferred to Wood Green Crown Court from jail two months later, an attempt was made to break Eren out of the prison van.
Police managed to foil the plot but shot 28-year-old Jermaine Baker in the process.
Last year during an inquiry into the fatal shooting of Mr Baker, a Met Police briefing note made public set out what the officers believed about Eren’s background to be.
Officers described Eren as “a senior member of a Turkish crime group” and said at the time of Mr Baker’s death he had “returned to the UK in breach of a deportation order having been sentenced for drugs trafficking offences”.
The gang, known as the Tottenham Turks, had a long-running feud with the rival Hackney Turks, police said.
They added the feud had seen “numerous shootings and murders dating back to 2009 in both London and Turkey,” the report added.
“Izzet Eren is wanted in Turkey for murder,” the document said.