Greek shares began trading on time on Tuesday, unaffected by a leftist trade union protest that sought to disrupt the session by blocking the entrance to the Hellenic Exchanges building.

The Athens bourse's benchmark index <.ATG> opened 0.55 percent higher on Tuesday and was trading at 1,961 points with gains of 0.23 percent at 0837 GMT.

Trade at the Greek bourse is electronic and officials said they had activated a backup system as key staff that operate the main system were not able to enter the building. About 150 unionists were protesting against austerity measures.

(Reporting by George Georgiopoulos)