In a television address on Saturday evening, Ben Ali said the election campaign "has been conducted within the framework of the law and democratic principles".
"The law will be applied ... with rigour against anyone who spreads doubts or accusations about the integrity of the electoral process without providing hard proof," he said.
Mohamed Bouchiha of the PUP party and Ahmed Inoubli of the UDU are the main challengers.
Ahmed Brahim, a candidate from the Ettajdid, or "Change" party, sent ripples through the country by declaring during the campaign that he would run "equal to equal" against Ben Ali.
Speaking to Al Jazeera from Tunis on Sunday, Nejib Chebbi, president of the opposition Progressive Democratic Party, said government banned him from taking part in the election because "this regime doesn't support free and fair elections".
"For 40 years we had a single candidate, and since 1999 the constitution has been amended in a manner that lets the president choose his own competitors," he said.
Asked about the strength of the candidates challenging Ben Ali, Chebbi said: "None of them can challenge the president because two of them are stooges and the third has no means to meet the people; his manifesto has been seized; his posters have not been displayed for one week."
|Ben Ali, seen with wife Leila, is credited with
sound economic leadership [GALLO/GETTY]
Ben Ali is backed by virtually every labour union and his party, the Constitutional and Democratic Rally or RCD, has been continuously in power since Tunisia's independence from France in 1956.
Ben Ali was last re-elected in 2004 with more than 94 per cent of the vote - a drop from his previous victories, which oscillated between 99.2 and 99.7 per cent.
Under the current constitution, he can run for president for the last time as the age limit is set at 75.
Ben Ali's opponents accuse him of suppressing dissent, although many voters credit him with making the North African country one of the most prosperous in the region and overseeing political stability.
Reporters without Borders has been very critical of the treatment of journalists who are being prevented from covering the election.
Human-rights groups have levelled accusations of abuse against Tunisia, which enjoys strong diplomatic and business ties with the US and Europe.