Ukraine's prime minister and a liberal challenger took the top spots in a presidential election on Sunday and will take their debate over the country's future through to a run-off next month, exit polls said. Two polls, conducted by Ukrainian research institutes, put Western-leaning Viktor Yushchenko ahead of Prime Minister Viktor Yanukovich, who is backed by Ukraine's establishment oriented towards neighbouring Russia. But both leading candidates fell short of the 50 percent needed to win outright in the first round. According to one poll, in which respondents concealed their identity, Yushchenko scored 45.2 percent to 36.8 percent for the prime minister. Another survey, in which voters were identified, gave Yushchenko a tighter lead, 41.98 percent to 40.11. Election observers said many voters, wary after a bruising campaign, were reluctant to reveal their identities. Official returns from the contest, with 24 candidates in the running, were to start coming in later on Sunday. The election, the most hotly contested of post-Soviet times, offered Ukrainians the stark choice of intensifying traditional links with Moscow or gradual movement towards the West. --More 2321 Local Time 2021 GMT