Zambian opposition leader Hakainde Hichilema has been arrested and detained by police on "treason charges", according to his lawyer Jack Mwiimbu.
The BBC reports Hichilema has not yet been formally charged. "We are definitely challenging the police action", lawyer Mwiimbu is quoted as saying.
It is understood that potential charges against Hichilema could be linked to the fact he blocked Lungu's motorcade on Sunday (9 April), according to an earlier statement from the president's spokesman, Amos Chanda.
Hichilema's party, the United Party for National Development (UPND) has denied any wrongdoing.
Zambian police forcibly entered the UPND president's residence to arrest him on the morning of 11 April. Hichilema was transferred from Woodlands Police station to Lilayi Police College after recording a 'warn and caution' statement, his party said on Twitter.
The government is yet to comment on Hichilema's claims.
Hichilema and his deputy Geoffrey Mwambawas were charged with 'sedition' in October 2016 and were granted bail. At the time, his team alleged the move was an attempt by the ruling party to silence dissenting voices.
The 54-year old unsuccessfully ran for the top seat five times in 2006, 2008, 2011, 2015 and 2016. Hichilema still contests losing to Lungu during August 2016's tightly contested election which was marred by allegations of rigging, with both sides accusing the other of inciting violence for political gain.
Following the elections in August last year, the government tightened its control over the media after the state broadcasting authority suspended broadcasting licences for three independent media houses it claimed had posed a risk to peace and stability during the poll.