By Camillus Eboh and Felix Onuah
ABUJA (Reuters) - The leader of Nigeria's Senate on Wednesday called for an investigation into a blockade of parliament by security forces and said it was an attempt to secure his impeachment but stopped short of saying he would run for president next February.
Senate President Bukola Saraki, Nigeria's third most senior elected official, last week defected from President Muhammadu Buhari's ruling party to the main opposition and is fast becoming a leading critic of the president.
He is the most high profile of the some 50 lawmakers who have left Buhari's party in the last few weeks, presenting a challenge for the president's bid to secure a second term at the helm of Africa's top oil producer.
The blockade by armed members of the Department of State Security (DSS) on Tuesday was the first such incident since the transition to civilian rule in 1999 and coincides with rising tension between lawmakers and the executive.
"We call for an investigation, and we demand that all perpetrators are brought to book," said Saraki during a news conference in the capital, Abuja.
He said the blockade was "an act of cowardice by those seeking to carry out an illegal impeachment of the leadership of the Senate in flagrant disregard of the law".
Saraki said the aim was to bar members of the opposition PDP so that a vote of lawmakers could be convened at which the two-thirds majority needed to impeach and replace him could be secured.
Defections have cost the ruling APC its majority in the Senate, parliament's upper house, presenting a challenge to Buhari's re-election bid by closing access to key patronage networks.
Saraki, when asked, refused to rule out running for president next year: "When the time is right, I will talk on that issue."
Yemi Osinbajo, usually vice president who is acting president while Buhari is in Britain on holiday, said the action by the DSS officers was "completely unacceptable" and was taken without the presidency's knowledge. He fired the DSS chief within hours of the blockade.
But, in an emailed statement issued on Wednesday, the APC appeared to row back from Osinbajo's criticism. It accused Saraki of plotting to "foment violence in the legislative chamber all in a bid to stop his impeachment".
"We are now aware that the timely intervention of the security operatives forestalled the planned violence," it said.
(Additional reporting by Afolabi Sotunde; Writing by Alexis Akwagyiram; Editing by Matthew Mpoke Bigg)