Rwanda's President Paul Kagame has promised that next month's elections will be free, following recent criticism from human rights groups.
"Rwandan voters have the freedom to decide," he said as he launched his campaign for the 9 August poll.
Several opposition critics have been killed or attacked recently - one senior official was buried as Mr Kagame was speaking.
The government has denied involvement in the killing.
The BBC's Geoffrey Mutagoma in Kigali says thousands of Mr Kagame's supporters - many in his party colours of red, white and blue - filled up the Amahoro National Stadium for his rally, and more were unable to get in.
Three other parties have launched their campaign but they are all seen as close to the president - two have been in government since 1994.
Several other political parties have been blocked from taking part in the elections.
Speaking to journalists before the rally, Mr Kagame said he thought his party would retain power: "I'm very confident that Rwandans will choose to work with RPF but I don't take anything for granted."
But BBC East Africa correspondent Will Ross says the election campaign could hardly be getting off to a more inauspicious start.
Just hours before the first campaign rally got under way, Democratic Green Party Vice-President Andre Kagwa Rwisereka was buried.
His almost decapitated body was found in Butare in the south of Rwanda last week after he was reported missing.
His party cannot compete in the elections, and it claims that the government has blocked its registration.
There have been other worrying events in recent months, including the murder of a prominent journalist, the attempted assassination of a former army general, and numerous arrests amid reports of splits within the Rwandan military, our correspondent says.
The government denies it has moved to crush the opposition and points to a smear campaign.
"We certainly might not be a model government for a lot of people, but we're not a stupid government, and we will not try to kill three people in a row right before election - an election in which we believe strongly that President Paul Kagame would win," Foreign Minister Louise Mushikiwabo told the AP news agency.
But our correspondent says not everyone is convinced.
United Nations Secretary General Ban Ki-moon has called for a full investigation into what human rights groups say are politically motivated attacks.
"In my view these are not elections," former speaker of the Rwandan parliament and government critic Joseph Sebarenzi told the BBC.
"Elections suppose competition and in Rwanda today you don't have that competition because the leaders of political parties have been put in prison, other political parties were prevented from presenting their candidate because they were not registered. Those who are running are friends of Kagame," he said.
But this was strongly denied by Rwanda's High Commissioner to London Ernest Rwamucyo, who said that the rules to register for elections were clearly set out.
"All those who fulfilled the requirements have been registered and are standing for election... They will be free and fair," he told the BBC.
He also denied that the other candidates were government stooges.
In the first election since the 1994 genocide, seven years ago, Mr Kagame won more than 90% of the votes.
He is also expected to win easily this time, for what is due to be a final seven-year term in office.
Mr Kagame has many admirers in the international community who praise him for rebuilding Rwanda following the 1994 genocide.
Some of his supporters say he is just the no-nonsense military strongman the country needs, given that Rwanda lies in such a troubled neighbourhood.
But our correspondent says allegations of oppression are not going away, and some analysts worry just how sustainable the status quo is.