Some civil society groups have raised concern that some politicians want to destabilise the country if their camps don't win the March 4 elections.
Without mentioning names or political parties, Kenya For Peace with Truth and Justice said the letter written by a purported Mungiki adherents to Chief Justice Willy Mutunga is a prove that some outlawed groups are now operating openly.
On Wednesday, Mutunga showed a letter threatening him and other judicial officials against barring Jubilee's Uhuru Kenyatta and William Ruto from running for office.
"We are concerned that this may be the beginning of attempts to destabilise the country in the event that one party fails to secure power through legitimate means," said James Gondi, the Africa Centre for Open Governance programme advisor who read a statement on behalf of KPTJ.
"It seems that a section of the political establishment did not learn a lesson in 2008 and will not hesitate to drag Kenya back into abyss for their own selfish and cynical goals."
The group said the 2007-08 post-election violence was partly started because Kenyans had no trust in the Judiciary. He said the Judiciary now enjoys much confidence from the public and warned that the reforms made by Mutunga may be eroded by such threats.