Let no individual, group, candidate, or supporter imagine that cowardly and darkly acts such as these will cow us”
Willy MutungaKenya's chief justice
"As a judicial officer, he must surely know that a letter allegedly written by alleged supporters of our candidates would have no evidentiary value and no court would admit it into evidence," Mr Mbiuki said, in a statement.
"Recourse to sensationalism and melodrama is not befitting of the head of the judiciary."
Mr Mutunga said five judges had been attacked ahead of the election.
He also faced an illegal attempt by an immigration officer at the airport in Nairobi last week to block him from flying to neighbouring Tanzania, he said.
"It requires some courage, ignorance, or political patronage or a combination of all three to stop a chief justice from travelling," Mr Mutunga added.
Intelligence chief Maj-Gen Michael Gichangi later phoned to apologise for what he called a "small hiccup", Mr Mutunga said.
"It has never happened on any of my numerous previous trips," he said.
"I have, therefore, concluded that this is deliberate harassment... I am convinced it is political."
Mr Mutunga said judges would not be held hostage by a "cabal of retrogrades".
"We shall decide all cases independently and with scrupulous fidelity to the constitution," he added.
"Let no individual, group, candidate, or supporter imagine that cowardly and darkly acts such as these will cow us."