"I have never wanted to use violence, but if I am stolen from, I will not give up the fight ... We are going to stage protests, we are not going to take this lying down."
The electoral commission's figures showed that Fabre scored about 692,000 votes, or almost 34 per cent of the vote.
Fabre said his party has proof the ruling party rigged the election in several ways, including intimidating opposition supporters and buying off other voters.
"The ruling party told our supporters that when they put their fingerprint on the ballot, they're going to be able to come and find them"
"The ruling party told our supporters that when they put their fingerprint on the ballot, they're going to be able to come and find them," he said.
"They gave money to buy people's consciences, there is fraud on a massive scale, we have the proof in our possession."
He vowed to present evidence of his claim in court.
In the immediate wake of the election, held on Thursday, both sides had claimed victory.
Then on Saturday police fired tear gas on some 200 protesters angry that the opposition party was trailing, Abalo Assih, a police spokesman, said.
The vote was seen as a key test of democracy in a region that in recent weeks has seen a coup in Niger and street riots over delayed elections in Ivory Coast.
International observers said the poll had gone smoothly, despite some procedural flaws. More than 3,000 local and nearly 500 European and West African observers monitored the election.
Gnassingbe, first took the presidency in 2002, after 38 years of authoritarian rule under Gnassingbe Eyadema, his father.
Tensions have risen in Lome as the president's supporters have warned they are willing to fight back.
"They accuse us each time that we stole their votes, threatening to pour on the streets," said Evariste Adoul, a pro-Gnassingbe activists.
"We shall show them that we also can take to the streets."
Election officials have been trying to prevent a situation similar to Togo's presidential election in 2005 when hundreds of people died in post-election violence.
The violence that followed that disputed vote left up to 800 dead according to various sources, but the UN put the toll at 400 to 500 deaths.
Yet parliamentary elections two years later were peaceful, raising hopes of an end to Togo's long history of political violence and leading to the restoration of foreign aid.