Cord leader Raila Odinga wants the Sh6.8 billion Northern Water Collector Tunnel project stopped as it is a "secret, ridiculous" Jubilee initiative.
The project funded by the World Bank seeks to divert water from sources in Aberderes to Thika's Ndakaini Dam for use in Nairobi.
"The secrecy surrounding this project is by design. Even the Jubilee government knows it's a bad project," he told journalists at his Capitol Hill office in Nairobi on Monday.
"...The same Jubilee that has made it a common practice to commission projects that had already been commissioned by the Grand Coalition Government so as to look productive has chosen to launch this project."
Raila said Jubilee conceptualised, commissioned and commenced construction of the project without establishing the impact of the project on the environment.
"This is unconstitutional, criminal and blatant abuse of power and grievous derogation of the rights of people affected," he noted.
He added that it will be one of the most disastrous initiatives as it will turn Muranga, Garissa, Ukambani and Tana River Delta regions into deserts, within five years of completion.
The project, which kicked off in March and is expected to end next December, is supposed to improve water services in Nairobi, Kiambu and Murang'a counties.
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Raila said a three-diameter tunnel will be dug high up in Aberdare range to connect all the seven to Thika's Ndakaini dam.
"A sum of Sh1.3 billion was paid to the contractor of the Murang'a secret project. The project will turn counties into desert," said the opposition leader.
"Secret water projects will lead to depletion of underground water resources affecting those who depend on Tana River.
Raila added that the tunneling of the project in Murang'a poses danger since the area is prone to mudslides and seismic activity.
The tunnel faced strong opposition from Murang'a residents and political leaders who expressed fears that it might deplete underground water and cause earthquakes.
Among those who opposed the project was Senator Kembi Gitura who claimed the county already supplies 75 per cent of the water consumed in Nairobi yet Murang’a residents have nothing to show for it.
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The Cord chief further said the project, which is part of Vision 2030, was not presented to Parliament for discussions before the government embarked on it.
"We oppose the project as public participation was not done," he said.
Further noting that no licence was obtained from water authorities before the project commenced, Raila said there were other ways of harvesting water.
"Nairobi needs water but there are other ways of solving the problem like harvesting water through dams," he said.
The tunnel was projected to boost Nairobi with about 700,000 cubic metres of water a day by completion.
According to the masterplan, 1.2 billion litres of water will be required daily by 2035 and 670 million litres 2017.
Philip Gichuki, Nairobi City Water and Sewerage Company managing director, said the county gets 580,000 cubic metres daily against a demand of 750,000 cubic metres, leaving deficit of about 200,000.
Water CS Eugene Wamalwa said in March that the project will proceed as controversies surrounding it had been resolved.
The Athi Water Services Board dismissed claims the tunnel will reduce water levels in Murang’a county rivers, saying the collector was approved by the Cabinet and Treasury.
He said the county cannot undertake a project that would hurt residents.
More on this: Athi board says Murang'a project won't reduce water levels in rivers
Also read: Water ministry working on solutions to Nairobi water deficit - CS Wamalwa