ANKARA (AFP) - Turkish Prime Minister Recep Tayyip Erdogan kicked off his presidential campaign on Saturday by promising to transform the role into a more powerful position if he wins the August polls.

"We are taking the first step of a new beginning", he told thousands of cheering supporters in mass rally in the northern city of Samsun, just days after announcing his candidacy.

A boisterous crowd of supporters from his ruling Justice and Development Party (AKP) cheered, called Mr Erdogan's name and waved Turkish and party flags under the pouring rain.

Mr Erdogan declared his candidacy on Tuesday to become the country's first democratically elected president and is widely expected to sweep to an outright victory in the first round of the polls due on August 10.

The 60-year-old leader made clear on Saturday that he would transform the largely ceremonial post held by Abdullah Gul into a more powerful position.

"Is it possible to be head of the state and sit back?" Mr Erdogan said, pledging to use any extra powers granted to him as president to push ahead with new airport and road construction projects.

"I am not leaving you. I am not stopping to serve. I am not having a break in our journey. I am not retiring for rest," he said.

"On the contrary I am nominated to a higher post to serve you, my glorious nation, and country better." His address was often interrupted by supporters shouting slogans like "Turkey is proud of you!"

The prime minister symbolically chose the city of Samsun to kick off his campaign. It was in there that Mustafa Kemal Ataturk, the founder of the modern Turkish republic, started the Turkish war of independence in 1919.

"Ninety-five years later, we are kicking off presidential campaign from Samsun, a new step for a new Turkey," he said.

Mr Erdogan, who has dominated Turkey's politics for over a decade, has steered his AKP government to successive election wins, notching up a bigger share of the vote each time.

But a series of crises over the past year ranging from mass street protests to a vast corruption scandal have badly bruised his stature both home and abroad.

Hailed by supporters as the man who brought stability to Turkey, the prime minister has also faced criticism from secular segments of the country for his uncompromising stance and heavy-handed tactics used to crush protests.

He is now bracing for a gruelling campaign schedule packed with rallies across the country. The largest is expected to be in Istanbul on July 11.

But as the campaign gets underway, the opposition called for Erdogan Saturday to resign from his post as prime minister in order to allow candidates to race under equal terms.

"As Erdogan declared his candidacy, he must show the virtue of stepping down as prime minister without causing an unfair competition and wasting the state means. This is a requirement of respect and obedience to democracy," said Mr Devlet Bahceli, the leader of the Nationalist Movement Party (MHP).

"The prime minister who holds a presidential campaign through virtual inauguration ceremonies must rally in squares under equal terms," he said, accusing Erdogan of being driven by "authoritarian motives and a desire for a one-man rule".

The Aug 10 vote is shaping up as a two-horse race between Erdogan and opposition candidate Ekmeleddin Ihsanoglu, a devout intellectual who has none of the charisma of the combative Erdogan.

Referring to Mr Erdogan as a polarising figure, Mr Ihsanoglu said an understanding that pits people against each other should not ascend to the presidency.

"The top of the state is not a place for frustration, partisanship, or alineation," he said.

Another candidate, Selahattin Demirtas, was put forward by the pro-Kurdish HDP party, and is expected to struggle to break into double figures on polling day as many Kurds are expected to vote for Erdogan.