A large block of hardline leftwing officials has left Greece's dominant Syriza party, highlighting the challenges Prime Minister Alexis Tsipras faces ahead of national elections next month.

A total of 53 members of the 201-strong central committee — the main party organ — announced their resignations Wednesday. They all back the Popular Unity party formed by disaffected Syriza officials. The party, led by former Energy Minister Panagiotis Lafazanis, is currently the third-largest in parliament with 25 former Syriza lawmakers.

Tsipras resigned last week after seven months in power, seeking a stronger mandate in elections expected Sept. 20.

His move followed a party rebellion triggered by his acceptance last month — against Syriza's key policy commitments — of new austerity measures to secure Greece a vital third international bailout.