Zambian police arrested several leading opposition figures on the night of national elections last week, accusing them of posing a threat to state security, the government said.
Votes were still being counted on Monday from Thursday's presidential and parliamentary elections, with partial results showing President Hakainde Hichilema in the lead.
The election is widely seen as a test of whether Hichilema's economic gains since a pandemic-era debt crisis are enough to win him a second term in the ?major copper producer.
His main rival, opposition leader Brian Mundubile, said heavily armed personnel had raided his residence after the election in what he described as an attack on his life.
The government said in a statement released late on Sunday that Mundubile had been at the scene when 11 people were apprehended, and that shots had been fired from the premises at law enforcement agents, resulting in an exchange of fire.
It said individuals had ?been found "in possession of high-grade military weapons, ammunition and other materials for use in armed insurrection" and that they had been under surveillance for months.