* Police fire teargas during sixth day of protests
* Rare for elite to question president's actions
* Rights groups say at least 50 killed in protests (Recasts with petition)
By Khalid Abdelaziz and Ulf Laessing
KHARTOUM, Sept 28 (Reuters) - Islamists and members of Sudan's ruling party called on President Omar Hassan al-Bashir on Saturday to cancel deeply unpopular austerity measures, ...