Venezuelan politician Henrique Capriles has registered as the presidential candidate for a coalition of more than 30 opposition parties in polls due in October.
After leading a march through the streets of the capital, Caracas, Mr Capriles vowed to fight crime and root out corruption.
President Hugo Chavez, who is recovering from cancer, is due to register on Monday.
He is running for a third term.
Mr Chavez has kept a low profile since returning from treatment in Cuba on 12 May.
But on Saturday Mr Chavez, 57, said recent exams showed he was now healthy and fit to run for another six-year term.
He vowed to attend in person to the Electoral Office, dismissing rumours that he would register online to avoid a public appearance.
Hugo Chavez has been in Cuba several times for cancer treatment
His 39-year-old opponent marched and jogged for 10km (6.2 miles) from a park in eastern Caracas to the electoral office.
Mr Capriles, who last week stepped down as governor of Miranda state to run for president, criticised the left-wing policies of Mr Chavez.
He promised to follow the example of former Brazilian president Luiz Inacio Lula da Silva, offering a balance of social programmes and pro-business policies.
"I aspire to become the president of all Venezuelans. I won't let them down," said Mr Capriles.
"I am not the enemy of anyone, I am the enemy of the problems, of the violence, I am the enemy of a country whose government prevents us from going forward."
The BBC's Sarah Grainger in Caracas says Mr Capriles has managed to get the backing of more than 30 parties, but it is still unclear whether that will be enough to defeat Mr Chavez, who was first elected in 1999 and remains popular.
His campaign manager, Jorge Rodriguez, said he expected "a huge sea of people" outside the electoral office on Monday when Mr Chavez turns up to register for the 7 October election.
Fecha de la Noticia:
10 de Junio de 2012