Updated

Police have banned protests across most of central Athens as German Chancellor Angela Merkel is due to pay a brief visit to Greece, a day after the crisis-hit country returned to international bond markets.

Unions and the left-wing main opposition party, Syriza, are backing protests planned Friday outside the cordon, to be manned by some 5,000 police officers.

Security was tightened further after a powerful car bomb exploded early Thursday outside the Bank of Greece, causing damage but no injuries.

During her visit, Merkel plans to meet fellow conservative Prime Minister Antonis Samaras who is facing opposition demands to hold early elections along with next month's vote for the European Parliament.

Thursday's five-year bond sale — the first since 2010 — raised 3 billion euros ($4.14 billion).