Court premises in the Zambian capital, Lusaka, and elsewhere have been shut as the deadline for filing a petition against the outcome of the recent presidential election approaches.
An unusual police presence has been reported at some courts, which have been taped off to prevent access.
Opposition candidate Brian Mundubile said he would challenge the election result, citing irregularities, after coming second with a 38% share of the vote. Incumbent Hakainde Hichilema won a second term with 60%.
Mundubile is in hiding over alleged threats made against him, amid a post-election period marked by controversy, including the shooting dead of an opposition figure and the arrest of others.
Monday is the last day on which petition against the presidential election result can be filed. Such a petition could potentially affect the inauguration of Hichilema, which is scheduled for 1 September.
On Monday, lawyers who arrived at the courts were being asked to depart.
One lawyer told the BBC that three armed men approached his vehicle, identified themselves, and instructed him to leave the area.
The courts that have been sealed off include the High Court in Lusaka, the Supreme Court, which also houses the Constitutional Court, and several local courts.
It is unclear when the premises will reopen. The BBC has contacted the Zambian police and the judiciary for comment.
Local media cite a directive by a judiciary administrative official telling all staff not to report to work following the closure of courts for "security reasons".
The memo, which has also been shared online by some opposition figures, does not give details of the security situation.
Some commentators have raised concerns over the closure while questioning the independence of the judiciary.
A prominent voice for the opposition, Emmanuel Mwamba, alleged the action was aimed at preventing Mundubile from filing his presidential election petition.
The Law Association of Zambia (LAZ) said it was concerned by the closure of the courts and the presence of heavily armed military personnel around court premises in Lusaka.
It said there was no formal public explanation for the closure nor its expected duration.
Referring to the "unsigned memorandum" from the judiciary addressing staff, it said no further official information had been provided to lawyers and the public.
"The closure of the courts has immediate implications for access to justice, particularly for persons requiring urgent judicial intervention," it said in a statement
It said the timing was "particularly significant in the present electoral context", citing a "seven-day period" in which a person may "petition the Constitutional Court to nullify the election of a President-elect".
Despite the court closure, preparations for Hichilema's inauguration appear to be continuing.
The youth and sport ministry has issued a media invitation for journalists to accompany a cabinet official on an inspection of arrangements at National Heroes Stadium, where the ceremony is expected to take place.
Mundubile last week rejected the presidential election results and said he planned to challenge the outcome through the courts.
He remains in a secure location under the protection of what his campaign describes as an "international human rights body".
He went into hiding after the arrests of opposition figures and the death of former minister Mutotwe Kafwaya during a raid by security officers, in what officials described as an exchange of fire in a security operation in the capital.
The arrests as well as the death of Kafwaya prompted calls from rights groups and the Law Association of Zambia for an independent investigations.
Amnesty International described the Kafwaya's killing as "deeply alarming" and called for prompt, thorough and impartial investigations. The lawyers' association similarly expressed "grave concern" and demanded investigation.
The election day, 13 August, was described by regional and international observers as largely peaceful, but they raised concerns about late electoral reforms, media freedom and access to the media in the run-up to the poll.
The EU observation mission said restrictions on the opposition may have contributed to an "uneven contest".
On vote tallying, the mission described a heavy military presence as intimidating and said it had received reports of attacks on some tallying centres.
It said staff in many centres did not record results on tally sheets as soon as they were announced, and cited weak verification between digital entries and paper records. It added that returning officers frequently paused proceedings while awaiting instructions from headquarters.
It said the mission also called for "maximal transparency" over the poll outcome, noting that the electoral commission did not publish separate polling-station results.
