* Kenyatta sworn in for second term in November
* Odinga speech gives no details of his plans
* Event is lose-lose for Odinga and Kenyatta - analyst
* Police presence light during Odinga's event (Adds U.N. comment)
By George Obulutsa and Maggie Fick
NAIROBI, Jan 30 (Reuters) - Kenyan authorities suspended television and radio stations on Tuesday as supporters of opposition leader Raila Odinga watched him take a symbolic presidential oath in a Nairobi park in a direct challenge to President Uhuru Kenyatta.
Security forces made no move to stop the ceremony, which authorities had said would be illegal, but the government later declared the opposition "National Resistance Movement" a criminal group, paving the way for potential arrests.
The movement is a loose grouping led by Odinga and other lawmakers that tried to rally support in November for a boycott of some products whose owners it says are aligned with government interests.
Odinga's supporters say he is Kenya's legitimate leader and Kenyatta's election was neither free nor fair.
Kenyatta's victory in August was annulled by the Supreme Court over irregularities but he won a re-run, which Odinga boycotted because some electoral commission reforms he demanded did not take place.
Kenyatta was sworn in for a second term in November and state institutions report to him.
"I, Raila Omolo Odinga, do swear that I will protect the nation as people's president, so help me God," Odinga, who held a bible, said to the cheers of more than 15,000 people in Uhuru Park, next to Nairobi's main business district.
During a speech lasting less than five minutes, Odinga declined to give details of his plans and said they would be disclosed in "due course". In a possible sign of division within the opposition alliance, Odinga's vice presidential candidate and two other senior leaders were absent.
Odinga said the vice president would be sworn in at a later date.
The attorney-general had warned that Odinga could be charged with treason if the event went ahead - an offence that can carry the death penalty.
As people assembled, authorities forced independent television and radio stations reporting on the gathering off air, several outlets said - the most widespread censorship for a decade.
The United Nations was watching developments in Kenya closely and it was "critical for the media to be able to operate freely and to report freely," U.N. spokesman Stephane Dujarric told reporters in New York.
He said the United Nations was urging "that all Kenyan parties, political actors and their supporters maintain a lawful and peaceful social and political environment."
"We have an illegitimate government," Odinga told a local broadcaster by phone in an interview that was streamed online before the event. "Whenever there is a crisis, there is also an opportunity. This is an opportunity to bring the country together."
Odinga refused to be drawn on his plans for after his symbolic inauguration, repeatedly telling the interviewer, "Hold your horses" and "We'll see".
Many of the protesters chanted pro-Odinga slogans, waved tree branches and blew horns and whistles. Afterwards several said they could not hear Odinga's brief address.
"I'm happy this has happened but we are waiting for the next step forward but we don't know what it is," said nurse Grace Waithera. "I'm very fed up with this ... government. If we can remove them we will."
Many of those at the rally had come from the capital's slums. Odinga has strong support there and in the west and along the coast, areas where people have long felt ignored by central government and shut out of political patronage networks.
"This is a lose-lose situation for both Kenyatta and Odinga and it was an entirely preventable crisis. It's not clear where Odinga goes next from here," said Murithi Mutiga, a senior analyst for the International Crisis Group.
"Kenyatta as well - by resisting all attempts at dialogue, he has put himself in a position where he will continue to struggle to be seen as the president of all Kenyans."
Although the police had said they would prevent any illegal assembly, there were no uniformed police in the park and no anti-riot officers or vehicles were visible.
Nearly 100 people were killed over the prolonged election period, mostly in clashes between Odinga supporters and police.
By 10:20 a.m. (0720 GMT), a number of independently owned media outlets including Citizen radio and television, NTV and KTN said that authorities had forced them off air.
A Communications Authority spokeswoman said it would comment later.
On Monday, Linus Kaikai, chairman of the Kenya Editors' Guild, said editors had been warned by authorities that they could be shut down if they covered the event. (Additional reporting by Humphrey Malalo and Katharine Houreld in Nairobi and Michelle Nichols at the United Nations; Writing by Katharine Houreld; Editing by Matthew Mpoke Bigg and James Dalgleish)
Our Standards: The Thomson Reuters Trust Principles.