"If we do not have any other way in the absence of a neutral justice, we will take to the street and will ask the international observers to determine our true number," Mohammad Qahtan, spokesman of the Common Forum, the opposition umbrella group that sponsored Ali Abdullah Saleh's main challenger, said on Friday.
But he insisted he was "not inciting violence" in a country which has one of the highest rates of private gun ownership in the world with an estimated three firearms for every one of its 20 million inhabitants.
"We will go into the streets stripped of all weapons, even the traditional dagger" carried by Yemeni men, he said.
Qahtan described as "laughable" partial results released by the electoral commission giving Saleh 80 per cent of the vote to 20 per cent for opposition standard-bearer Faisal bin Shamlan.
Out of 4.33 million votes cast, Saleh received nearly 3.45 million, while Bin Shamlan got nearly 885,000, electoral commission spokesman Abdo al-Janadi said, citing results reported from polling stations and candidates' representatives.
The official Saba news agency said the figures represented the results from 17,500 ballot boxes out of a total of 27,000 allocated for the election.
But "these results are not definitive," Janadi stressed.
The opposition had said on the eve of Wednesday's election that it feared supporters of the incumbent, in power for 28 years, might be tempted to falsify the results.
But the European Union for Elections Observations Mission has described the presidential and local municipal elections in Yemen as a genuine contest, Aljazeera said.
"The elections presented a notable opportunity in the region for an incumbent head of state to face a real challenge at polls,” the EU EOM preliminary report said.
The head of the mission, British peer Baroness Nicholson of Winterbourne, on Thursday said the election - only the second presidential vote since north and south Yemen united in 1990 - "in general" met international standards.
Nicholson said: "Generally, we saw that voting procedures were conducted very well in 82 per cent of the polling stations we visited countrywide, in spite of a number of irregularities such as breaches of secrecy and underaged voting.
"Our assessment is good or very good. Those were credible and peaceful elections."
Nearly 90,000 troops and police were deployed to oversee voting by 9.25 million eligible electors, 3.9 million of them women.
Five people were killed and six wounded in clashes between Saleh's supporters and opposition rivals, prompting voting to be suspended in about a dozen of the 5,620 polling stations, officials said.
The run-up to the vote had also been marred by violence, including sporadic clashes between rival supporters, the deaths of well over 50 people in stampedes at Saleh campaign rallies and a foiled attack on oil installations.