Production at Libya's El Feel, Sharara and 108 oilfields has stopped after tribal leaders protested against the apparent arrest of a former finance minister.
The shutdown was confirmed by engineers working at the sites.
The closures are being led by the Al Zawi tribe against the disputed arrest of Faraj Bumatari on Tuesday.
The protesters released a video statement from the 108 oilfield saying they had closed a valve.
"We affirm the continuation of the oil closures and we may escalate the situation to more than that if our son Faraj Bumatari is not released," footage showed a man surrounded by others saying.
No protesters have entered the Sharara oilfield so far, two engineers working in the field told Reuters on Thursday, but the threat from the demonstrators was enough for production to be halted.
The Sharara field, one of Libya's largest production areas with a capacity of 300,000 barrels per day, has been a frequent target for protesters fuelled by political causes.
The site is in the Murzuq basin in the south-east and is run by state oil firm NOC via Acacus alongside Spain's Repsol, France's Total, Austria's OMV and Norway's Equinor.
El Feel field, with a capacity of 70,000bpd, is operated by Mellitah Oil and Gas, a joint venture between NOC and Italy's Eni.
No immediate comment was available from NOC.
Oil-rich Libya was plunged into over a decade of chaos after a Nato-backed uprising toppled and killed dictator Muammar Qaddafi in 2011.
The country has since been politically divided, with one administration in Tripoli and another in the east backed by Gen Khalifa Haftar.
The leader of Al Zawi tribe, Al Senussi Al Ahlaiq, told Reuters the closure of El Feel was aimed at pressuring authorities in Tripoli to release Mr Bumatari, who was reportedly arrested by security agents after arriving at Mitiga Airport on Tuesday.
The tribe, to which Mr Bumatari belongs, claims he was abducted.
Mr Al Ahlaiq said preparations were under way to close Tripoli's entire water supply.
Mr Bumatari is a candidate for the position of central bank governor, the tribe said in a written statement, which "makes him vulnerable to danger and kidnapping".
The UN mission in Libya said the "shutdown must be immediately ended".
The mission said five members of Libya's High State Council had also been banned from travelling at the same airport.
"These acts create a climate of fear, promote tension between communities and tribes," said the UN Support Mission in Libya (Unsmil).
