Esmail Bakhshi "was arrested last night in cooperation with security and law enforcement forces," Mansour Mohammadi, the prosecutor general of Dezful, in Khuzestan province, told the judiciary's news website Mizan Online.
Bakhshi was one of the organisers of weeks of protests at the Haft Tapeh sugar factory in the city of Shush in November and December, over unpaid wages and alleged criminal activity by new private owners.
State TV on Saturday broadcast a programme claiming that Bakhshi and Sepideh Gholian, another activist who supported the Haft Tapeh strikers, had connections with Europe-based activists who "aim to topple the state".
The programme featured footage of Bakhshi and Gholian sitting behind desks in front of red and blue curtains, detailing their connections and activities with the activists, allegedly based in several European countries.
The footage was undated and taken in an unknown location.
Both Bakhshi and Gholian were detained last year during the protests.
Gholian was also arrested Sunday, according to Mizan Online.
The semi-official Fars news agency, close to conservatives, said Bakshi had attempted to flee the country to continue the "torture allegation project" abroad.
After his release from his first detention, Bakhshi claimed on his Instagram account, that he had been tortured during his 25-day detention by agents of the intelligence ministry.
In the post in early January, he also said the ministry had been eavesdropping on him and his family.
His Instagram account was later deleted.
The torture claim sparked a controversy in Iran, where officials from members of parliament to high-ranking judicial figures promising a full investigation.
Enquiries by parliament, the judiciary and the intelligence ministry found that Bakhshi had not been tortured.
Iran saw multiple strikes and protests last year over working conditions in key sectors including steel, education, mining and transport.
The Haft Tapeh protests ended in late December, with the workers being paid and the factory re-opening.
In November, the head of Iran's judiciary warned restive workers against creating "disorder".
Mizan Online quoted Ayatollah Sadegh Larijani as saying: "workers should not allow their demands to become an excuse and an instrument for the enemy."