Nigerian police and security forces filtered access to parliament on Tuesday amid a surge of tension between President Muhammadu Buhari and political rivals.
Hooded armed men from the police and Department of State Service (DSS) took over the entrance of the two chambers - the Senate and House of Representatives - from around 7:00, preventing workers, journalists and other visitors from entering.
Lawmakers who were initially denied access were later allowed into their offices, but others were barred from entering. Parliament began a two-month recess on July 24.
Police and the DSS did not return repeated calls asking for an explanation for the high-profile operation.
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Independent sources speculated it was a show of strength connected to a political crisis ahead of presidential and parliamentary elections next February.
Lawmakers loyal to Buhari's All Progressives Congress (APC) are believed to be planning to oust the president of the Senate, Bukola Saraki.
Saraki, the country's third-highest ranking politician, last week dumped the APC for the main opposition Peoples Democratic Party (PDP).
The APC has called on him to resign, suspecting him to be the force behind defections that have damaged the party's standing and Buhari's chances of re-election.
On the last day before recess, 14 members of the 109-seat senate and 37 members of the 360-seat House of Representatives left the APC.
Party primaries are due between this month and October, coinciding with mounting discontent at Buhari's style of government and handling of widespread violence across the country.
Atiku Abubakar, who was vice president from 1999 to 2007, condemned the police operation as an "illegal and undemocratic attempt by fascist forces.. to force a change in the Nigerian Senate's leadership."
Atiku, who is a PDP presidential hopeful, urged Buhari to call the police and DSS to order "and abandon this madness."