Detention follows dispute over Hichilema’s vehicles allegedly blocking President Lungu’s motorcade on main road.
Zambian police have denied the main opposition party’s claim that its detained leader Hakainde Hichilema was denied access to his lawyers and family members.
“We have not blocked lawyers and Mr Hichilema’s members of the family from visiting him because that is his constitutional right,” police spokeswoman Esther Mwaata-Katongo said.
Hichilema’s lawyer Jack Mwiimbu, however, said that he had seen his client on Wednesday but had since been prevented from visiting him.
“There are instructions for him not to be seen by anybody without authority from police headquarters. We have therefore started processing court papers. We want the court to compel the police to allow us to see our client,” Mwiimbu told Reuters news agency.
Mwiimbu said Hichilema had complained on Wednesday of feeling unwell after police fired tear gas into his house when he was arrested.
Treason is a non-bailable offence in Zambia, with a minimum jail term of 15 years and a maximum sentence of the death penalty.
The two political leaders are old rivals, with Lungu beating Hichilema narrowly in two presidential elections. Lungu, the Patriotic Front leader only just won last August’s vote, which the opposition United Party for National Development (UPND) says was rigged.
On Saturday, Lungu passed through Mongu, 500 km west of the capital Lusaka, and his motorcade was obstructed because Hichilema refused to give way, police said.
“The opposition leader disobeyed police orders to give way to the presidential motorcade in an attempt to put the life of the republican president in danger,” the police chief told reporters in Lusaka on Wednesday.
Zambia’s economy has been depressed for years by low commodity prices, mine closures, rising unemployment, power shortages and soaring food prices that Hichilema blames on mismanagement by Lungu.
Hichilema has launched several legal attempts to challenge the result of the August election, which he lost narrowly.
He says that the vote was rigged and accuses Lungu of political repression in Zambia, which has been known for its relative stability.
The election campaign was marked by clashes between supporters of Lungu’s Patriotic Front (PF) party and the UPND.
Hichilema, a wealthy self-made businessman, has run unsuccessfully for president five times.
The UPND called on supporters not to protest on Thursday as it said the government was seeking a chance to impose a state of emergency.
Some supporters at the press conference carried placards reading “Release HH before Zambia goes into flames”.