In Manisa, crowds shouted: "Turkey is secular and will remain secular. Count how many of us there are, Tayyip [Turkey's prime minister]."
"We have seen this theatre many times... [there is] no way other than democracy"
Srimedya, Bursa, Turkey
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Political tension is running high after a warning from the pro-secular army against Gul and a court decision to annul the first round of parliamentary voting for head of state.
Recep Tayyip Erdogan has come under heavy pressure in the run-up to presidential elections, with secularists fearing that he would expand his party's control by appointing Gul, the country's foreign minister
The pressure led Erdogan to call for early parliamentary elections, which are scheduled for July 22.
A measure is also being debated in parliament to allow the president to be elected directly by the people, rather than by parliament, which is dominated by members of Erdogan's party.
Gul said in an interview with The Financial Times that he would secure a majority if a popular vote was held to decide who should be the country's president.
Speaking to the newspaper on Saturday, he was critical of the court that decided on Tuesday to annul the first-round parliamentary vote on the presidency.
Gul said: "As the foreign minister, I respect and observe the court's decision, but that doesn't mean I am happy with it."
He referred to the crisis over the presidency, which prompted the military to threaten to intervene and protect the country's secular order, as a "shadow".
Gul said: "Our responsibility is to remove this shadow and to put everything on the right path."
Meanwhile, two Turkish parties merged on Saturday, which could strengthen the opposition against the ruling AK party in July's general election.
The ANAP and True Path parties, which have 20 and four seats respectively in the 550-seat parliament, announced the merger at a joint news conference and said their new name would be the Democrat party.