Turkish Foreign Minister Abdullah Gul has pledged to adhere to secular principles if, as expected, he is elected president.
Abdullah Gul is expected to be elected president
PM Recep Tayyip Erdogan announced on Tuesday that Mr Gul had been named the governing AK party's candidate.
The decision came after thousands had taken to the streets to urge Mr Erdogan not to stand.
Secularists fear that a president from the AK - a party with Islamist roots - could undermine Turkey's secular order.
Mr Gul insisted that "the president must be loyal to secular principles", adding: "If I am elected I will act accordingly".
Both Mr Erdogan and Mr Gul have wives who wear the Islamic headscarf - a highly divisive issue in Turkey.
Mr Gul defended the headscarf choice on Tuesday, saying "these are individual preferences and everybody should respect them".
Parliament will hold the first round of voting on Friday and the AK's majority means its candidate is likely to win.
Turkey has been a republic since 1923, with a strict separation of religion and the state.
The AK party has its roots in political Islam.
But correspondents say that Mr Gul is seen as less confrontational than Mr Erdogan.
Turkey's chief of staff, Gen Yasar Buyukanit, and outgoing President Ahmet Necdet Sezer have urged the new president to defend Turkey's secular values.
Speaking to the AK group in parliament, Mr Erdogan said Mr Gul was "the person who emerged at the end of our evaluations as the candidate to become Turkey's 11th president".
There will be several rounds of voting in the 550-member parliament before the new head of state takes office on 16 May.
Mr Gul, 56, has steered Turkey's European Union accession talks since becoming foreign minister in 2003.
He had a brief spell as prime minister after the AK party's election victory in November 2002.
Educated in England as well as Turkey, he is an English speaker and is regarded as a moderate, the BBC's Pam O'Toole reports.