A TOUGH election battle began in Sai Kung yesterday, as six candidates shaped up for the race to fill the Regional Council seat vacated by convicted politician Gilbert Leung Kam-ho. Although nominations do not close for another week, campaigning is already underway, with a rare clash between the territory's three liberal parties and Hongkong's new pro-Beijing political force about to face its first test at the polls. After Leung's three-year sentence for bribery, the Independent Commission Against Corruption (ICAC) has vowed to keep a tight watch on this and other elections. ''We will be monitoring the elections carefully,'' said its new commissioner, Bertrand de Speville. Hongkong's political parties are using the August 8 by-election as a dry run for the 1995 polls. Already past alliances between liberal groups have been forgotten as the United Democrats of Hongkong, Meeting Point, and the Association for Democracy and People's Livelihood (ADPL) have all entered the race. ''We made no attempt at compromise before putting forward our candidate,'' said United Democrat nominee Lam Wing-yin, vice-chairman of the party's New Territories East branch. City Polytechnic political analyst Jane Lee Ching-yee said the in-fighting showed the former allies realised it was no longer enough to just say they were liberals. Ms Lee said the trio - who also included Meeting Point's Wai Hing-cheung and ADPL vice-chairman Law Cheung-kwok - would concentrate on fighting for votes in the fast-growing new town of Tseung Kwan O. Meanwhile the pro-Beijing Democratic Alliance for the Betterment of Hongkong (DAB), which has never faced the polls before, hopes the Sai Kung ballot will enhance the party's image. ''Whatever the result will be, we must gain something from this election,'' its chairman, Tsang Yok-shing, said. DAB treasurer Cheung Hon-tin is standing for the seat. The other candidates are Sai Kung District Board members Wong Shui-san - who has strong support in rural areas - and Chow Yin-ming. Meanwhile, Mr de Speville said he had noted concern over corruption in the wake of the Gilbert Leung case, and the ICAC was conducting a thorough review of how to tighten safeguards - with reference to Legislative Council functional constituency elections.