Complex sentence: Estimated population of Maardu as of 1 July 2005 was 16,490, of whom the majority are Russians, with Estonians reaching only a 20% of the population: one of the lowest (if not the lowest) rate in Central and Western Estonia.
Split into simpler sentences: Estimated population of Maardu as of 1 July 2005 was 16,490. The proportion of Estonians was one of the lowest (if not the lowest) in Central and Western Estonia.