At least one million Turks have rallied in the city of Izmir to protest against any government plans to undermine Turkish secularism.
The major demonstration was the fourth since the ruling AK Party nominated Foreign Minister Abdullah Gul as its candidate for the country's presidency.
Many Turks suspect Mr Gul of having an Islamic agenda, something he denied before withdrawing his candidacy.
Parliamentary elections scheduled for November are now to be held in July.
The AK Party - which has roots its in political Islam - argues that a general election will only prove that it commands widespread public support.
The government has also backed a constitutional reform package that would allow the president to be directly elected by the people.
The BBC's Sarah Rainsford, in Istanbul, says an AK Party candidate would be likely to win any presidential vote.
There were loud calls at the latest rally for opposition parties to unite and weaken the AK Party at the polls.
But there is still little sign of that on the left-wing and little time now left, our correspondent says..
Opinion polls show the AK Party remains the most popular in Turkey, despite the mass protests
The demonstration in Izmir - Turkey's third-largest city - was the fourth mass gathering in favour of the republic in as many weeks.
Buildings and streets in Izmir were covered in flags and portraits of Kemal Ataturk, the founder of the republic.
Boats flying the flag also joined the demonstration in Izmir's harbour.
Many of the protesters said they wanted to make a statement
The main slogan, once again, was that Turkey is secular and will stay secular, our correspondent says.
Many of those rallying were clear that their main aim was to make a clear statement to the government.
"The AK Party will most likely win the elections but they will have to take into account these demonstrations and what people are saying," said Haluk Berk, a doctor and university teacher.
"These rallies have been useful in forcing the government to take a step back," protester Neslihan Erkan told the Reuters news agency.
"The danger is still not over. These rallies must continue until there is no longer a threat."
Many secularists consider the government's efforts to confirm Mr Gul as president as evidence of a hidden Islamist political agenda.
Nationalist opposition parties boycotted two parliamentary votes on the issue.
The military, which regards itself as the guardian of Turkey's secularism, has also indicated its opposition to the AK Party's move.