"Gayoom presents himself as a reformer, but he's changed colour like a chameleon," Nasheed said.
"He has a track record of a dictator. He has jailed opponents on the most ridiculous charges," he said.
Other prominent opponents include Hassan Saeed, a former attorney-general, and Ghaseem Ibrahim, a local businessman.
Results from the election, in which 208,000 people are eligible to vote, are due on Thursday. Voting will last for 12 hours.
The vote marks a sea change in Maldivian politics, given that previously it was illegal to criticise Gayoom, who presided for six terms under a one-candidate system.
Cluster of 1,192 islands scattered across 800km of Indian ocean off southern India
Population 300,000, mostly ethnic Dravidians, Indo-Aryans, Sinhalese and Arabs
Official religion: Islam
Official language: Dhivehi, although English and Arabic are widely spoken
Economy is dependent on tourism with growth averaging seven per cent
Timeline: Maldives political milestones
"But I hope we can push him out gracefully," Anni said, calling Gayoom a "has-been sultan" who had bought up votes in the election.
The Maldives has the highest income per head of population in south Asia, largely due to tourism. Hotels on the archipelago charge up to $15,000 a night.
However, there is poverty in the country and high unemployment particularly among women and young men, and increasing drug use, worsening crime and a chronic housing shortage are threatening to undermine economic progress.
A bomb attack carried out by Islamic fighters in the capital Male wounded a dozen tourists last year, the first such attack in the country.
The tsunami of 2004 also adversely affected the economy, which is highly dependent on tourism.
Gayoom has concentrated his campaign on outlying islands, home to more than half the electorate and where he appears to have the most support.
However, opposition rallies in Male have drawn widespread support.
The winning presidential candidate must secure more than 50 per cent of the vote and avoid a second-round run-off.
So far, there has been no fair indication of how the election will pan out, given the lack of reliable opinion polls conducted before the vote.
"I feel I must be at the helm to see through the reform programme," Gayoom said on Tuesday.
He criticised opponents for casting him as a politician similar to Robert Mugabe, the Zimbabwean president, or Fidel Castro, the former president of Cuba.
"It is very wrong to compare me to those people," he said, while pledging to respect the result of the vote.
The vote comes in the same week that a report by the Maldives independent auditor general alleged that the government had given millions of dollars to its supporters.