MOSCOW, May 18 — The police detained more than a dozen opposition leaders and journalists here on Friday, preventing them from flying to a provincial city to take part in anti-Kremlin protests. The action followed a wave of harassment of the protest’s organizers.

The detentions added a new element of frostiness to President Vladimir V. Putin’s meeting with Chancellor Angela Merkel of Germany and other leaders of the European Union near the provincial city, Samara.

“Speaking frankly, I would like that those who today want to hold a demonstration in Samara would be able to express their opinion and have the chance to do so,” Ms. Merkel said, displaying a willingness to criticize the Russian leader far more openly than her predecessor, Gerhard Schröder.

Among those detained at a Moscow airport was Garry Kasparov, the former chess champion, who was on his way to the latest in a series of protests organized by Other Russia, a protest group he leads.