Complex sentence: The 2009 Ashura riots broke out on 27 December 2009 in Iran (mainly in Tehran) against the outcome of the June 2009 Iranian presidential election which they claim was rigged, the demonstrations were part of the 2009 Iranian election protests and were the largest since June.
Split into simpler sentences: The 2009 Ashura riots were held on 27 December 2009 in Iran against the outcome of the June 2009 Iranian presidential election, which demonstrators claim was rigged. The demonstrations were part of the 2009 Iranian election protests and were the largest since June.