:Tens of thousands of protesters marched through the streets of Istanbul on Sunday (April 29) to protest against the candidacy of Foreign Minister Abdullah Gul in the country's presidential vote. Waving red and white Turkish flags, the protesters chanted slogans against Gul and Prime Minister Tayyip Erdogan, denouncing them as a a threat to Turkey's secular order separating state and religion. The protesters are throwing their support behind the army, long viewed in Turkey as the ultimate guardian of the secular republic. "Turkey is secular and will remain secular," the protesters chanted on Caglayan Square in Turkey's largest city. Police estimated the crowd size at 200,000. Secularists are worried by Gul's Islamist past and the fact his wife wears the Muslim headscarf. Turkey currently bans the headscarf from universities and public offices. Turkey's secularist elite, including army generals, fear Gul and the ruling AK party could tamper with the strict separation of state and religion in the overwhelmingly Muslim country. Protesters called on the government to resign. But speaking to reporters in Ankara on Sunday, Gul refused to withdraw from the presidential vote. Gul, architect of Turkey's European Union membership bid, said he would remain his party's candidate. A second round of voting in parliament, where the AK Party has a majority, is set for Wednesday (May 2). The third round is set for May 9. The Istanbul rally mirrored a similar one in Ankara two weeks ago against Erdogan running for president. After that protest, Erdogan nominated the more conciliatory Gul instead. But the main secularist opposition party has asked the Constitutional Court to rule the presidential election invalid. The court says it will try to deliver its verdict by Wednesday. If the strongly secularist court upholds the opposition appeal, Erdogan must call an early parliamentary election. Outgoing President Ahmet Necdet Sezer would remain in office until a new parliament could choose his successor. If the court backs the government, the presidential election process would continue. The EU, which began accession talks with Turkey in 2005, and the United States, Ankara's NATO ally, have both called for a democratic and constitutional resolution of the crisis.