ISTANBUL, May 14 (Reuters) - Turkey headed for a runoff vote after President Tayyip Erdogan led over his opposition rival Kemal Kilicdaroglu in Sunday's election but fell short of an outright majority to extend his 20-year rule of the NATO-member country.

Neither Erdogan nor Kilicdaroglu cleared the 50% threshold needed to avoid a second round, to be held on May 28, in an election seen as a verdict on Erdogan's increasingly authoritarian path.

The presidential vote will decide not only who leads Turkey but also whether it reverts to a more secular, democratic path, how it will handle its severe cost of living crisis, and manage key relations with Russia, the Middle East and the West.

Kilicdaroglu, who said he would prevail in the runoff, urged his supporters to be patient and accused Erdogan's party of interfering with the counting and reporting of results.

But Erdogan performed better than pre-election polls had predicted, and he appeared in a confident and combative mood as he addressed his supporters.

"We are already ahead of our closest rival by 2.6 million votes. We expect this figure to increase with official results," Erdogan said.

With almost 97% of ballot boxes counted, Erdogan led with 49.39% of votes and Kilicdaroglu had 44.92%, according to state-owned news agency Anadolu. Turkey's High Election Board gave Erdogan 49.49% with 91.93% of ballot boxes counted.

Thousands of Erdogan voters converged on the party's headquarters in Ankara, blasting party songs from loudspeakers and waving flags. Some danced in the street.

"We know it is not exactly a celebration yet but we hope we will soon celebrate his victory. Erdogan is the best leader we had for this country and we love him," said Yalcin Yildrim, 39, who owns a textile factory.