Krystian Zimerman, the great Polish concert pianist, is usually a man of few words. He doesn't, as a rule, talk to the audience during performances. He says little or nothing in the press between his all-too-rare concert tours - not even about his habit of travelling everywhere with his own Steinway grand piano. He rarely grants them the pleasure of an encore.

So he triggered more than the usual rumble of discomfort when he raised his voice in the closing stages of a recital at Los Angeles' Disney Hall on Sunday night and announced he would no longer perform in the United States in protest against Washington's military policies.

"Get your hands off my country," Zimerman told the stunned crowd in a denunciation of US plans to install a missile defence shield on Polish soil. Some people cheered, others yelled at him to shut up and keep playing. A few dozen walked out, some of them shouting obscenities.

"Yes," Zimerman responded with derision, "some people when they hear the word military start marching."

According to Mark Swed, the Los Angeles Times's veteran classical music critic who witnessed the incident, Zimerman hesitated before deciding to speak up. He was about to strike up the first notes of the final piece on his programme, Karol Szymanowski's Variations on a Polish Folk Theme, when he "sat silently at the piano for a moment, almost began to play, but then turned to the audience".

Swed said he delivered his tirade "in a quiet but angry voice that did not project well".

Zimerman appears to have been upset by Barack Obama's decision, announced this month, to maintain the Bush-era policy of installing a missile defence shield in Poland and the Czech Republic.

Obama insisted the shield was part of a defensive posture against Iran, not Russia, and that he intended to remove it as soon as the threat from Iran subsided. But many Poles have accused the US of wanting to mount a military occupation of their country, and fear the shield could make them a target of Russian aggression.

Classical musicians are not exactly famous for political ranting. When Beethoven became disillusioned with Napoleon after he crowned himself emperor in 1804, the loudest thing the composer did was remove the dedication to him from his Eroica symphony. Leonard Bernstein spoke out against the Vietnam war and hosted a notorious fundraiser for the radical Black Panther movement, but never discussed his politics from the conductor's podium.

Zimerman, though, has developed something of a track record - especially since the 9/11 attacks. In 2006 he announced he would not return to the United States until George Bush was out of office. The same year, at Baltimore's Shriver Hall, he prefaced his performance of Beethoven's Pathetique sonata with a denunciation of America's prison at Guantánamo Bay.

At least some of his opprobrium appears to be personal. Shortly after 9/11, his piano was confiscated by customs officials at New York's JFK airport, who thought the glue smelled funny. They subsequently destroyed the instrument.

For several years he chose to travel with just the mechanical insides of his own piano and install them - he is a master piano repairer, as well as player - inside a Steinway shell he borrowed from the company in New York. In 2006 he tried to travel with his own piano again, only to have it held up in customs for five days and disrupt his performance schedule.