Meles' ruling Ethiopian People's Revolutionary Democratic Front (EPRDF) is widely favoured to win a third consecutive five-year term at the helm of Africa's top coffee producer.
But five main opposition parties and coalitions have set aside their differences to form one of the strongest coalitions yet to challenge Meles.
Meles told the cheering, singing crowd estimated by witnesses and party officials to number about one million: "The huge turnout is an indication that EPRDF and its policies are supported by the people. We seek your support for re-election on May 15. We urge you to vote in a peaceful manner."
Meles has been asking Ethiopians not to vote for the opposition saying they were promoting divisive ideologies similar to those of the ethnic Hutu militias who massacred hundreds of thousands of minority Tutsis and Hutu moderates during the 1994 genocide.
Opposition parties are campaigning to change the constitution to remove an article which grants the right for any of the nine ethnically based federal states that make up Ethiopia to secede, saying it undermines unity.
Meles says his movement saved the country from disintegration when it introduced the provision after ousting Marxist dictator Mengistu Haile Mariam in 1991, warning that removing the right to secession would enflame ethnic tensions among Ethiopia's dozens of tribes and linguistic groups.
A malnourished Ethiopian boy watches as food is donated by the U.S. Agency for International Development at Dodota distribution centre near Harar, in eastern Ethiopia May 6, 2005. The United Nations Children's Fund (UNICEF) says it needs $54.6 million, from which $13 million is needed within the next 60 days, to save the lives of up to 170,000 severely malnourished children and to feed 360,000 moderately malnourished children in Ethiopia. As many as 300,000 Ethiopian children will die from malnutrition this year if donors do not come forward with food aid and money, Bjorn Ljungqvist, UNICEF's head in Ethiopia said on Wednesday. REUTERS/Andrew Heavens