Security forces temporarily blocked the entrance to Nigeria’s parliament on Tuesday, preventing lawmakers and others from entering for up to an hour, witnesses said.

Witnesses said armed men wearing the black uniform of the Department of State Security stood at the gates of the building, where they were later joined by police officers.

A Reuters witness said security agents, some of whom were masked, then allowed lawmakers to enter but continued to bar government agents and journalists.

A senior aide to Senate President Bukola Saraki was among a number of people who tweeted pictures and video from outside the National Assembly building. One showed the arrival of police outside the building and another showed a man, described as a senator, talking to a man flanked by an armed man.

A Nigeria Police Force spokesman could not immediately be reached for comment.

Saraki, whose Senate role makes him Nigeria’s third most senior politician, is among a group of lawmakers who have quit President Muhammadu Buhari’s ruling party over the last few weeks and joined the main opposition.

The defections precede presidential and legislative elections early next year.

Buhari plans to seek a second term in the February 2019 presidential vote but the loss of influential figures and divisions within his party could cost him support from powerful patronage networks and among voters.