Zambian opposition leader Brian Mundubile has been charged with treason, his lawyer has said, amid heightened political tensions in the country.

Mundubile was arrested on Thursday, after a deadly raid by the authorities led the opposition politician to go into hiding.

He had disputed the results of the country's recent presidential election, in which incumbent President Hakainde Hichilema secured a second term.

The authorities then accused Mundubile's NRPUP party of planning an armed insurgency, which it denied.

On Saturday night, Mundubile's lawyer Milner Katolo announced that his client had been charged with treason. The opposition leader looks forward to answering the accusation in court, Katolo told the Zambian Eye.

The lawyer said 17 others, including Mundubile's running mate Makebi Zulu, had also been charged with treason.

Mundubile gained 38% of the vote in the 13 August election, while Hichilema secured 60%, official results show.

The day after the poll, security forces raided a property in the capital, Lusaka, linked to Mundubile, killing a former cabinet minister and arresting a number of other politicians.

The government said they seized weapons during the raid.

Following the raid, Mundubile and Zulu took refuge at a United Nations building in the city. The pair were detained after the president put pressure on the UN to hand them over.

The opposition was unable to mount a legal challenge to the election outcome because the courts were closed by the authorities on Monday, the final day a case could be filed.

Hichilema is due to be sworn in for a second term on 1 September.

Regional and international observers described election day as largely peaceful, but Zambia's Human Rights Commission has called for accountability for people whose whereabouts remain unknown following arrests and security operations during the elections.