Turkish President Recep Tayyip Erdogan declared victory Sunday in a historic runoff vote that posed the biggest challenge to his 20 years of transformative but divisive rule.

The vote, cast as being the most important in generations, saw Turkey's longest-serving leader pitted against a powerful opposition alliance.

The 69-year-old leader overcame Turkey's biggest economic crisis in generations and the most powerful opposition alliance to ever face his Islamic-rooted party to take an unassailable lead.

Turkey's main cities erupted in jubilation as Erdogan spoke.

Traffic on Istanbul's iconic Taksim Square ground to a halt and huge crowds gathered outside his presidential palace in Ankara.