Ruaraka Member of Parliament TJ Kajwang has been arrested outside Milimani Laws Court by plain clothes officers.
The MP administered the 'oath of office' to Nasa leader Raila Odinga on Tuesday at the Uhuru Park grounds.
He was taken to Directorate of Criminal Investigations headquarters along Kiambu Road.
He is representing MPs in the case in which legislators are challenging reduction of their salaries as well as allowances by the Salaries and Remuneration Commission.
Mr Kajwang had just appeared before High Court judge George Odunga for a ruling.
Mr Kajwang' and another lawyer, Miguna Miguna, were at the centre of events at Uhuru Park that culminated in the "swearing-in" of Mr Odinga as the 'people's president'.
During the ceremony, Mr Kajwang' donned a lawyers' robe and wig and received a thunderous welcome by Nasa supporters.
As Mr Odinga took the "oath", Mr Kajwang' stood right behind him, although his role remained unclear.