"Odinga is the one we recognise as the president and that is why we are swearing him in", said hairdresser Benta Akinyi standing near other opposition supporters blowing horns and whistles.
The event may be marred by violence, with police saying that the gathering is unlawful as they have received no notice of a registered political event.
Kenyan opposition leader Raila Odinga of the National Super Alliance (NASA) raises his fist before taking a symbolic presidential oath of office in Nairobi, Kenya January 30, 2018.
The Kenya Editors Guild, however, protested the move noting that the country had competent media professionals who make decision on what is news and what is not.
Mr. Odinga's supporters say he is Kenya's legitimate leader and Kenyatta's election was neither free nor fair.
The Civil Society Reference Group (CSRG) described the shutdown as "the worst State interference with the freedom and independence of the media in post-independence Kenya".
'Kenya is on a very slippery trajectory in regard to human rights, and president Kenyatta urgently needs to reverse this trend, Namwaya said.
"Today is a historic day for the people of Kenya", Odinga said in a speech after taking his oath.
Authorities had repeatedly warned that such an inauguration would be treasonous and that Odinga could face arrest.
Kenya's Attorney General Githu Muigai declared in December that swearing in an alternative president would constitute an act of treason for everyone "facilitating the process", without naming Odinga specifically.
Hon Raila Amolo Odinga and H.E. Stephen Kalonzo Musyoka as President and Deputy President of the people respectively is on as has been announced earlier.
He said they are investigating why the Government resorted to such actions, adding that if it is because of NASA "oathing" coverage the State was overreacting. It also said it ruled against Kenyatta because the commission refused to open its computer system for court scrutiny to dispel Odinga's claims. Kenyatta was declared the victor of the original August 8, 2017 poll, but his victory was annulled due to electoral malpractices, and the courts ordered a re-run of the election on October 26. Following the boycott, president Uhuru Kenyatta was declared the victor, with over 98% of the vote, granting him a second term in office.
Since boycotting the re-run poll, citing a lack of reform at the election commission, the National Super Alliance (NASA) strategy has been to challenge Kenyatta's legitimacy by seeking to establish parallel government structures.