Kenya's opposition leader Raila Odinga, who boycotted last year's disputed presidential elections and accused his rival of electoral fraud, on Tuesday proclaimed himself the "people's President" at a non-sanctioned swearing-in ceremony.

The event brought together tens of thousands of his supporters prompting the government to shut down TV broadcasts and issue warnings of high treason, the BBC reported.

National Super Alliance (NASA) frontman Odinga refused to recognize his rival Uhuru Kenyatta as the legitimate President ever since the latter rushed to victory with 98 per cent of the vote in the controversial October 2017 election re-run.

The vote, boycotted by the opposition, had been slated by the Supreme Court after it found irregularities in the original ballot two months prior.

"Today's step is one step towards doing away with electoral autocracy and to establishing proper democracy in our country," Odinga said, as he took "oath" before the huge crowds of his supporters gathered in Uhuru Park in central Nairobi.

"I promise to be loyal to the people and the Republic of Kenya, that I will preserve, protect and defend the constitution of Kenya," the NASA leader added. 

Several television channels transmitting images live from the ceremony, including Citizen, KTN and NTV had their broadcasts taken off air by the government's communication's authority, although several live streams were still accessible online.

President Kenyatta had earlier warned the media not to cover Tuesday's event and Attorney General Githu Muigai said holding such a ceremony amounted to "treason".

Speaking earlier to KTN, Odinga said the media ban "confirms we have descended to the level of Uganda", which stopped media coverage during elections in 2016.