A top opposition leader denounced the provisional results released by the Ethiopian elections board, but did not indicate what action his party would take.
Prime Minister Meles Zenawi arrived midmorning at the main square in Addis Ababa and addressed the crowd as hundreds of blue-uniformed federal police stood guard.
"We'll not boast about our victory, but we'll add to the burden of work that we have to deliver for the next five years," said Meles, who seized power in a 1991 coup.
The European Union mission said Tuesday that the weekend vote had been peaceful, but that there were inequalities that favored the ruling party leading up to the election.
Thijs Berman, the EU's chief observer, said they had received a number of complaints of violence and harassment.
"The sheer volume and consistency of these complaints is a matter of concern," Berman told journalists, noting Ethiopia's lack of a national voter list. "These shortcomings lead us to the conclusion that this electoral process falls short of certain international commitments," he said.
US-based Human Rights Watch also had criticized Sunday's vote, saying voters were told they could lose food assistance, public-sector jobs, loans and educational opportunities if they voted against the ruling party.
Ethiopian election officials said they witnessed no irregularities, and government spokesman Bereket Simon said the election was free and fair.
"It is sad to hear that while the election officials have said Ethiopians voted in a democratic way, Human Rights Watch, which has nothing to do with the election observation, is declaring our election fraudulent," he said. "The people of Ethiopia have voted, and no one is going to take their vote away from them." Government supporters criticizing Human Rights Watch held signs at Tuesday's rally that read: "We chose our leaders, accept the results" and "Stop second guessing us." However, several attendees were unable to read the English-language signs they held aloft.
Top opposition leader Hailu Shawel said the election results were not fair, and said that he suspected many of people who attended the rally were paid to do so and did not necessarily support the ruling party and its allies.
"The vote is 100 percent controlled by the ruling party," Hailu said.
Sunday's vote had been closely watched by international observers after the contentious 2005 election, in which the opposition won an unprecedented number of parliamentary seats only to endure police crackdowns and the killing of 193 demonstrators after the votes were counted.
Analysts had predicted an easy win for the ruling party, led by Meles, a US ally now poised to get five more years of power.
Since the violent elections in 2005, the opposition and some analysts say the government has systematically stifled the competition, while limiting the media and restricting aid groups from working on human rights issues.
Ethiopia is frequently criticized for its human rights record, including by the US State Department, which in a March report cited reports of "unlawful killings, torture, beating, abuse and mistreatment of detainees and opposition supporters by security forces, often acting with evident impunity." Still, the US considers Ethiopia an ally and provides billions of dollars in foreign aid. Both countries want to curb Islamist extremism in Somalia, Ethiopia's unstable neighbor to the east.
