A key security organ held a meeting late in the night on Monday and decided that police should be withdrawn from Uhuru Park and allow Nasa to 'swear in' Mr Raila Odinga.
During the meeting at Harambee House, the National Security Advisory Committee (NSAC) also decided to withdraw Mr Odinga's official police bodyguards.
The officers were, therefore, disarmed and ordered to report to their units.
The security detail of Nasa co-principals Kalonzo Musyoka, Musalia Mudavadi and Moses Wetang'ula were also withdrawn.
NSAC is usually chaired by the head of civil service and is only second to the National Security Council, which is chaired by the President.
In pulling police out of Uhuru Park, the venue of the Nasa ceremony, authorities relied on intelligence that the opposition leaders hoped a confrontation would arise between officers and their supporters.
In the event of a confrontation, the Oopposition planned to call off the 'swearing-in' ceremony while at the same time blaming the government, and particularly the police, for provoking their supporters.
NSAC agreed that past Nasa processions thrived on chaos in which lives were lost and property destroyed.
A source privy to the NSAC meeting -- but who we agreed not to name because they are not authorised to divulge the details to the media -- also said the opposition chiefs' bodyguards were recalled because "it would make no sense to provide State security for the purpose of engaging in an illegal activity".
But even as the security was withdrawn, other teams of undercover police officers were deployed to maintain round-the-clock surveillance around the opposition chiefs.
The source said Mr Odinga held a lengthy meeting with close confidantes at Dusitd2 Hotel, on Riverside Drive.
Among those in attendance were Siaya Senator James Orengo, Mombasa Governor Hassan Joho, Nasa strategist David Ndii, former Machakos Senator Johnson Muthama and controversial business magnate Jimi Wanjigi.
At some point, Mr Mudavadi was spotted at the hotel but left before the meeting ended.
Some of the opposition chiefs spent Monday night at the hotel, according to the source, who added that Nasa co-principals differed on the oath ceremony following pressure from the United States and European countries.
The Western governments threatened to have their names included in "no-fly" lists, meaning airlines flying to those destinations would not allow them to board their aircraft.
Being placed on no-fly list also means that the countries involved would revoke the visas of the individuals and their close relatives.