YEREVAN (Reuters) - The Central Elections Commission on Wednesday declared Armenia`s prime minister and acting president Robert Kocharyan the winner of the former Soviet republic`s presidential runoff election. "According to the preliminary data, we can already say that Robert Kocharyan has won," said commission deputy chairman Ruben Manukyan. Manukyan said Kocharyan was leading challenger Karen Demirchyan by about 59 to 41 percent of the vote with 99.4 percent of precincts counted. The commission said turnout was 68.1 percent of the country`s 2.3 million voters. Monday`s runoff in the mountainous Trans-Caucasus republic was held after Kocharyan and Demirchyan finished first and second respectively in a first round of voting on March 16. President Levon Ter-Petrosyan was forced to resign by opponents in February just 18 months into his second five-year term and special elections were held to replace him. Kocharyan`s chief spokesman, Aghvan Vartanyan, declared his candidate the victor. "We have won and won by a big margin," he said. Demirchyan campaign chief Armen Khachatarian blasted the election as illegitimate and repeated a list of abuses his side said occurred during voting. "This election was neither free nor fair," he told Reuters. The Organisation for Security and Cooperation in Europe (OSCE) said on Wednesday in a preliminary report the election had been marked by irregularities but that they were not serious enough to have an impact on the result. Some OSCE observers disagreed and said they thought the report had been far too soft. The Central Elections Commission is due to release final preliminary results by 10 a.m. (0500 GMT) on Thursday.