German Chancellor Angela Merkel told Greece the sacrifices it makes toward cutting spending will pay off, an unpopular message to tens of thousands who protested her visit to the country.

Ms. Merkel made her remarks after meeting Tuesday with conservative Greek Prime Minister Antonis Samaras in Athens. Her five-hour stop in Greece was her first visit to the country since the eurozone crisis began several years ago.

Thousands of police were spread throughout the capital to control more than 25,000 protesters. Police briefly clashed with some demonstrators and detained at least 40 people.

The protesters' anger stems from Greece's struggle to keep up with commitments needed to guarantee it will pay its debts and retain long-term euro membership. Mr. Samaras' government has pursued massive new austerity measures that are deeply unpopular.

Ms. Merkel has been vilified in Greece for her role in encouraging those measures, especially as Greeks struggle in an ongoing recession.