Complex sentence: More than 30,000 Polish citizens seek refugee in Karachi which was governed by the British Government and remained in Karachi, becoming the citizens of the newly established state, State of Pakistan, in 1947.
Split into simpler sentences: More than 30,000 Polish citizens sought refugee in Karachi, which was then governed by the British Government. Some remained in Karachi, becoming citizens of the newly established State of Pakistan in 1947.