Zambia's opposition leader and presidential election runner-up Brian Mundubile was in police custody and being questioned, state broadcaster ZNBC reported on Thursday.
This comes two weeks after a disputed but largely peaceful presidential election, where incumbent Hakainde Hichilema was declared the winner by authorities.
Mundubile had been in hiding after police raids and arrests the day after the election, saying he planned to challenge the results in court as the government accused him of national security offenses and plotting an insurgency.
Zambian police summoned him for questioning last week.
National broadcaster ZNBC reported that Mundubile and his running mate, Makebi Zulu, "have been handed over to police and are currently undergoing interviews."
It cited a police spokesman, Godfrey Chilabi, as saying the two opposition leaders were being interviewed by a team of law enforcement officers in the presence of their lawyers.
Chilabi told ZNBC that a statement would be issued once the interview process was concluded.
The broadcaster reported that the pair had been "under the protection of the United Nations" in the capital Lusaka prior to surrendering to police.
Police have also said that Mundubile was seeking refuge with an "international organization" earlier in the week, and said it was looking to question the opposition candidate and his running mate seeking to establish links between them and "various alarming activities... which have the potential to threaten or undermine national security."
Zambia voted on August 13, in an election that observers had expected to deliver a relatively clear mandate for a second term for incumbent President Hakainde Hichilema.
Mundubile briefly declared victory on election night, but Hichilema called on people to await the vote count and formal results.
The counting process was briefly suspended at one point, amid claims of attacks targeting election officials. The electoral commission ultimately found that Hichilema had won around 60% of the vote, albeit with Mundubile faring better than predicted.
Hichilema had made allegations of plots threatening national security shortly before the vote, and on the day after polling, police raided Mundubile's home and other opposition sites, arresting 11 people. A former cabinet minister, Mutotwe Kafwaya, was shot and killed during an exchange of gunfire in the operation. Mundubile went into hiding.
The leader of the National Reconciliation Party for Unity and Prosperity (NRPUP), which governed Zambia before Hichilema, continued to allege election irregularities and to dismiss the allegations against him as fabricated.
On Monday this week, on the legal deadline to issue a court challenge to the results, Zambia closed its entire judiciary citing "security reasons," drawing criticism from legal and rights groups.
Zambia typically has a reputation as one of Africa's more stable democracies but this month's election and its aftermath has not exactly reinforced this.
Hichilema is scheduled to be sworn in for his second term next week, on September 1.
