Zambian police have arrested opposition leader Hakainde Hichilema for committing "treason", according to his lawyer.
Hichilema was narrowly defeated by President Edgar Lungu in an August 2016 election, which he described as fraudulent. His attempts to mount a legal challenge have so far been unsuccessful.
The leader of the United Party for National Development (UPND) arrived at a police station in the capital, Lusaka, on Tuesday after officers raided his home overnight.
Authorities allege that Hichilema blocked Lungu's motorcade with his own convoy of vehicles during a traditional ceremony in Zambia's Western Province over the weekend.
They say Hichilema's motorcade did not yield after presidential security guards signalled for it to get out of the way. The opposition party has denied any wrongdoing.
"They have detained him for having committed treason along Limulunga road by obstructing the presidential motorcade," Jack Mwiimbu, Hichilema's lawyer, told the Reuters news agency.
"He has not yet been formally charged. We are definitely challenging the police action."
Police were not immediately available to confirm the charges.
Charles Kakoma, a UPND spokesman, said police broke doors before raiding Hichilema's house in an exclusive suburb of Lusaka.
He said police had blocked the access road to the residence but did not say what they were looking for.
"They broke the doors and tear-gassed the house," Kakoma said.
"They left the home after picking him."
Mwiimbu said Hichilema, a wealthy businessman known as "HH" who has run for president five times, was unwell after the raid.
"He is not well because of the tear gas. His wife fainted three times last night because of the tear gas," he said.
Treason is a non-bailable offence in Zambia, with a minimum jail term of 15 years and a maximum sentence of death.
"The PF party demanded action against HH after the motorcade situation," Neo Simutanyi, an independent political analyst, told the AFP news agency.
"HH is still trying to challenge the result through the courts, but Edgar Lungu is president in theory and in practice," he added.
"There is a lot of mistrust between the two sides.
Simutanyi said that under the treason charge, Hichilema "could be detained for some time, so this will suit the ruling party who want to silence the dialogue of the election result not being recognised".
Following last August's vote, Hichilema alleged polling irregularities and went to court, but judges rejected a petition to block Lungu's inauguration.
Last October Hichilema and UPND vice-president Geoffrey Mwamba were arrested for unlawful assembly and sedition when they tried to visit supporters in jail.
His team called the arrest an attempt by the ruling party to silence dissent.
Source: News agencies