President Zain al-Abidin bin Ali, in power for 17 years and accused by critics of seeking a presidency-for-life status, is widely expected to win a fourth five-year mandate on Sunday.
More than 4.6 million voters in the small North African country will also elect 189 members of parliament, with the ruling Constitutional Democratic Rally expected to keep a tight grip on the legislative house.
The rally, late on Friday night and into the early hours of Saturday in Tunis, was the largest held by the opposition in years, witnesses said, and the first time the small and fragmented opposition presented a unified front.
The opposition says general elections will make a mockery of the government's democratic pledges.
Enemies of transition
"The enemies of transition to a genuine democracy are trembling now. With us staying together, Tunisia will not be the same the day after the election, whatever the results," Muhammad Harmal, head of the secular al-Tajdid party, told the crowd.
Al-Tajdid is fielding Muhammad Ali Halawani, a 47-year-old university philosophy teacher, in the presidential race. He is one of three candidates from small opposition parties running against Bin Ali, 68, last re-elected in 1999 with 99.5% of the vote.
Human-rights groups say Tunisia has never had a free election since independence from France in 1956. The country has had just two leaders since then.
Bin Ali is seeking a fourth term
The leading opposition Democratic Progressive Party withdrew all its 89 candidates from the legislative election on Friday, saying the presidential and parliamentary poll would be a sham.
The government has dismissed opposition parties' accusations that authorities prevented some of them from campaigning, and restricted their access to voters through the state-run media.
Some opposition groups say Sunday's multiparty elections are part of a well-orchestrated government drive to put a veneer of democracy on a de facto single-party rule.
Diplomats say Washington is watching the election for signs the Tunisian government is opening up the political process. President George Bush urged bin Ali to carry out more reforms when they met earlier this year at the White House.
Many opposition activists hope the campaign will rally
Halawani (L) is the presidential
candidate for al-Tajdid party
grassroot support and unify an opposition often fractioned along blurry ideological lines.
"A fighting spirit is returning to the opposition camp after years of reluctance to join the political battle against this
government," leftist figure Mustafa bin Ahmad told the rally.
Halawani, in the closing speech of his campaign, said: "Whatever the results of this election, we'll stay together to rebuild a strong progressive opposition."