Speaking at the No To A Life Presidency rally, opposition parties rejected changes to the constitution that allow President Zain al-Abidin bin Ali to run for another term.
The leader of the Democratic Forum for Labour and Freedom, Mustafa bin Jaafar, called on a sizeable audience to boycott the 24 October presidential vote, saying the result was a foregone conclusion.
"Taking part in the presidential election is the equivalent of legitimising a coup de force against the constitution."
In a referendum in May, Tunisians "voted" massively in favour (99%) of amending the constitution to allow Bin Ali, who has been in power since 1987, to stand for a fourth five-year mandate.
They also voted to lift the age limit for presidential candidates from 70 to 75, meaning 67-year-old Ben Ali could run again in 2008.
Under the old constitution, the president was only allowed to remain in office for three terms.
Another opposition party led by Muhammad Najib Shabbi plans to field a presidential candidate, even though the government has barred such a move.
The official reason for preventing the Democratic Progress Party from presidential elections is because it does not hold any seats in parliament.
Shabbi said that "the rules of the game of democracy have been confiscated, with the government choosing the incumbent president's adversaries ahead of time".
The head of the Communist Workers Party, Hamma Hammami - who sprang to prominence when he was jailed in 2002 after he voluntarily came out of hiding to appeal against an earlier nine-year sentence handed down in absentia in 1999 - also called on voters to boycott the election.
Hammami vowed that the opposition alliance would work together "to create an alternative democracy" in Tunisia.
The opposition parties also said they were devising a coordinated campaign for legislative elections, due to be held at the same time as the presidential poll.