Following a political meeting in Bonn, Merkel said Monday that she expects "constructive and friendly discussions in the light of expectations that we have of Greece" when she meets with Samaras and other officials on the one-day visit.
Merkel's spokesman Steffen Seibert said the chancellor appreciates steps Greece has taken but will make clear "what still has to happen."
But he insisted that "with her visit, the chancellor is in no way anticipating what the troika will deliver."
"I cannot speak of presents here — everything that has to be decided will have to be decided when we have the foundation of data that the troika report will deliver," he said.
The main Greek private sector GSEE union deplored the police protest ban as "unprecedented, undemocratic", saying much of central Athens has been designated "a forbidden city."
More than 300 pensioners marched on European Union offices in Athens Monday, where they burnt an EU flag in protest of the latest round of austerity measures.
GSEE later also organized a protest rally outside parliament to be followed by another march Tuesday. About 2,000 people attended the peaceful rally, including Christos Velios, member of a financial activist group called Plan B, that supports Greek unilaterally cancelling part of its national debt.
"(Merkel's) presence here is an insult to the Greek people's feeling of national sovereignty," he said. "I don't really have anything to say to Mrs. Merkel. She is very good at her job. The question is: What are the Greek people doing?"
Geir Moulson in Berlin and Derek Gatopoulos in Athens contributed.