Waving a bible, opposition leader says 'I am president'
He told the crowd at the national stadium, who appeared to be in carnival mood, that he would now lead the country and improve their lives.
Hundreds of thousands of people have been celebrating on the streets of Antananarivo.
There has been no reaction as yet from the government - which had warned that it would take unspecified action - or from the incumbent president, Didier Ratsiraka, who maintains that the opposition leader's declaration is illegal.
Mr Ravalomanana, however, says he beat Mr Ratsiraka in elections last December but was defrauded of victory.
The government wants a second round of voting.
There have been weeks of strikes in support of Mr Ravalomanana in Antananarivo, where he enjoys widespread support.
But officials had said earlier that they would not let Mr Ravalomanana take power.
Opposition protests
"The affairs of state won't suffer any vague desires for a forceful takeover," Prime Minister Tantely Andrianarivo told journalists on Thursday.
The move follows weeks of opposition protests in Madagascar since the December elections.
Mr Ravalomanana won the ballot but officially not by enough to secure the presidency without a second round of voting.
He claimed the first round was rigged and vowed to take over the role of president.
At a rally on Thursday, there was some confusion as the other defeated presidential candidates urged him to put off his decision until next week.
Prime minister's warning
But the suggestion that he might comply with this request provoked a strong reaction from the crowd - which urged him to take control of the country straight away.
Earlier, the prime minister said he still believed in following the electoral process legally and constitutionally.
He warned that he might take some form of action if Mr Ravalomanana decided to assume the role of president.
It is not known what form such action might take - or how Mr Ravalomanana intends to try and legitimise his illegal, if popular self-appointed position.
Talks chaired by the Organization of African Unity to settle the stalemate over the elections and the widespread evidence of fraud failed to produce an agreement.