Zambian opposition leader Brian Mundubile was in police custody and being questioned, state broadcaster ZNBC reported.
The move came two weeks after a disputed but largely peaceful presidential election that authorities say incumbent Hakainde Hichilema won.
Mundubile had gone into hiding after police raids and arrests swept opposition strongholds the day after the election.
He had vowed to challenge the results in court, while the government accused him of national security offenses and plotting an insurgency.
Police said Mundubile and his running mate, Makebi Zulu, were being interviewed by officers in the presence of their lawyers.
The pair had reportedly been under United Nations protection in Lusaka before surrendering to police this week.
Official results gave Hichilema roughly 60% of the vote, and he is set to be sworn in for a second term next week.