|A million people demonstrated in Istanbul on Sunday in support of secularism [Reuters]|
Turkey's military, considered a custodian of secularism in the country, has also weighed in by voicing its concern against the possible spread of "political Islam".
"We have seen this theatre many times... [there is] no way other than democracy"
Srimedya, Bursa, Turkey
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Erdogan's ruling AK Party, buoyed by strong economic growth and the support of the European Union it aims to join, remains defiant.
Gul, architect of Turkey's attempts to gain EU membership, has refused to stand aside despite calls from protesters.
Turkey's constitutional court, meanwhile, has started hearing an opposition request to suspend the presidential election.
The court has said it will try to issue its verdict by Wednesday, when parliament is due to hold a second round of voting on Gul's candidacy. Gul is not expected to win the presidency until the third round on May 9.
Turkey's lira currency lost more than two per cent and Turkish shares fell six per cent on Monday due to the political turmoil.
On Sunday, as many as a million people attended an anti-government rally in Istanbul, Turkey's biggest city and business hub.
Many protesters accused the government of planning an Islamic state and criticised it for failing to consult the opposition over the choice of president, who carries great symbolic weight and has important veto and appointment powers.
Source: Al Jazeera and agencies