* Challenger Ouattara declared winner by election body
* Top legal body says results invalid
* Military seals land, air and sea borders
By David Lewis and Tim Cocks
ABIDJAN, Dec 2 (Reuters) - Ivory Coast sealed all its borders on Thursday after its election commission declared challenger Alassane Ouattara provisional winner of a presidential run-off, a result the top legal body rejected.
The Constitutional Council, the body that must ratify the result that judged President Laurent Gbagbo loser of the poll, said the commission's announcement was illegal.
Ivory Coast's military later sealed air, land and sea borders, without giving any reasons for the move.
The media regulator said it had suspended the signal for French broadcaster Canal Plus Horizon. Satellite channel France24 and Radio France International FM were also off air.
After repeated delays due to wrangling within his organisation over the results, election commission chairman Youssouf Bakayoko announced that Ouattara had won the Nov. 28 vote with 54.1 percent of the vote.
"The electoral commission has, in accordance with the law, handed over to the Constitutional Council the results it has received and validated, accompanied by the result sheets," Bakayoko said at a hastily-organised news conference.
Gbagbo's party, which wants ot cancel results from four northern regions that are Ouattara strongholds, rejected it.
Soon after the election commission statement, Paul Yao N'dre, a staunch ally of Gbagbo's who heads the Constitutional Council, said the poll body had missed a Wednesday deadline to issue provisional results.
He is widely expected to rule as the president wants.
"Once it's expired, the election commission is no longer authorised to announce results," he said on state television. "It is the Constitutional Council that is authorised to announce decisions on the contested results."
The council has seven days to give a final tally but N'Dre said final results could be expected in hours.
According to the election commission margin, the council would have to cancel nearly 400,000 votes to swing the result.
French President Nicolas Sarkozy urged the council to "respect the will clearly expressed by the Ivorian people" and the U.N. Security Council warned Ivory Coast that it was prepared to take "appropriate measures", a diplomatic codeword for sanctions, against anyone thwarting the electoral process.
Later, U.N. Secretary-General Ban Ki-moon said in a statement he "assures the people of Ivory Coast that the (U.N. mission) ... will undertake all possible actions, within its mandate, to help keep the electoral process on track, to preserve peace and security in the country."
The United States also urged all parties in the Ivory Coast to respect the results as proclaimed by the commission.
"Credible, accredited electoral observers have characterized the balloting as free and fair, and no party should be allowed to obstruct further the electoral process," White House spokesman Mike Hammer said in a statement.
The International Criminal Court issued a statement on Thursday saying it would be monitoring acts of violence.
The vote, delayed for five years, was meant to reunite the top cocoa grower, split in two by a 2002-2003 war, but has instead exposed north-south divisions and sparked violence.
Bakayoko surprised reporters by walking into the U.N.-guarded hotel in Abidjan which Ouattara has made his base and reading out results which had been blocked despite intense international pressure for them to be published.
Cheers erupted from Ouattara supporters gathered at the hotel, which has been placed under U.N. guard with a handful of armoured personnel carriers outside. Ouattara called on Gbagbo to stick by pledges he made before the poll to respect the results and said he planned a national unity government.
A Western diplomat told Reuters Bakayoko made the announcement in the hotel, rather than the electoral commission, because he feared for his personal safety.
There are widespread fears the electoral dispute will erupt into violence between Gbagbo's and Ouattara's youth supporters or between Ouattara's supporters and security forces.
N'Dre said that the Constitutional Council would analyse the results and announce a definitive winner. Earlier, a spokesman for the council said it had the power to annul results in some provinces and tally accordingly.
Gbagbo's party has accused the rebels still controlling the north of the country of intimidation and of rigging the poll for Ouattara. The rebels have denied the charge and Ouattara's party accused Gbagbo of blocking the results of a poll he had lost.
Tensions have spiked during the uncertainty.
Security forces shot dead at least four people at a Ouattara party office in an Abidjan suburb overnight, while members of Gbagbo's party said they had been attacked at their residence in the same suburb by Ouattara's militants, leaving some wounded.
An opposition leader said another 12 were shot dead by security forces in downtown Abidjan.
Fears of unrest pushed cocoa prices up over four percent in Thursday trade. Many Ivorian exporters have suspended business.
The yield on Ivory Coast's $2.3 billion Eurobond ticked slightly higher, reaching 10.68 percent compared to its pre-vote levels of below 10 percent. (Additional reporting by Loucoumane Coulibaly and Ange Aboa in Abidjan, Louis Charbonneau at the United Nations; writing by Mark John)