Unions and the left-wing main opposition party, Syriza, are backing protests planned outside the cordon, to be manned by some 5,000 police officers. Merkel has been among the most vocal supporters of the austerity measures that have helped heal Greece's public finances but proved painful for Greeks.
During a visit in 2012, the German chancellor had been greeted by mass anti-austerity protests that turned violent. Security this time was also tightened further after a powerful car bomb exploded early Thursday outside the Bank of Greece, causing damage but no injuries.
Merkel's visit comes as Greece this week reached a milestone in the recovery from its financial crisis — it successfully tapped bond markets for the first time since 2010, raising 3 billion euros ($4.14 billion) in five-year debt.
The country, however, still has far to go to heal its economy, which has shrunk by about a quarter during the crisis, and to reduce unemployment from around 27 per cent.
Syriza leader Alexis Tsipras in a newspaper article criticized the cordon, saying Merkel would not get a sense of the impact of the austerity measures she supported.
"I encourage her to visit a hospital and witness the third world conditions ... or a school where teachers try to cope with underfed children," he wrote.
During visit her Friday, lasting only several hours, 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.