Zambian police detained opposition leader Hakainde Hichilema on Tuesday after raiding his home, his spokesman said, but authorities could not immediately say why he was being questioned.
Hichilema, a wealthy economist known as "HH", was defeated by President Edgar Lungu in an election last August, which he described as fraudulent. His attempts to mount a legal challenge have so far been unsuccessful.
"HH is now at Woodlands police station awaiting processes there. They are questioning him now," Hichilema's media aide Clayson Hamasaka said in a statement.
Hichilema's lawyer Jack Mwiimbu said the opposition leader had been moved to Lilayi Police Training School in the capital for questioning.
"We have no idea what they want ... we are on our way to Lilayi. This is something strange. It has never happened before. He is not well because of the tear gas. His wife fainted three times last night because of the tear gas," Mwiimbu told Reuters.
A spokesman for Hichilema's United Party for National Development (UPND), Charles Kakoma, said police broke doors to the opposition leader's house in an exclusive suburb of Lusaka and fired tear gas canisters into the mansion. 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 teargassed the house," Kakoma said. "They left the home after picking him."
Neither police nor Hichilema were available to comment.
On Sunday, presidential spokesman Amos Chanda said Hichilema had obstructed Lungu's motorcade, saying Hichilema's motorcade maintained its lane instead of getting off the road.
Hichilema was granted bail in October after being charged with sedition, a move his team said was an attempt by the ruling party to silence dissent.
The opposition leader had been accused of assembly without a permit after he gave an impromptu address in the rural town of Mpongwe, police said.