Diplomats from nearly 30 countries have joined Kenya's opposition and media groups to condemn the raid by armed, masked police on a media group.
"These acts of thuggery have no place in an open democratic society," said a statement from the US embassy.
The police say they raided the Nairobi offices of a newspaper and a TV station because of incitement to ethnic hatred.
Thousands of copies of The Standard newspaper were burned and KTN was taken off the air for several hours.
The newspaper has been critical of President Mwai Kibaki's handling of recent corruption scandals.
The government has repeatedly accused The Standard of fabricating stories.
Three Standard journalists, arrested before the raids over a story about President Kibaki, have been charged with publishing alarming statements and released on bail.
Former colonial power Britain is one of the countries to condemn the raids as an "unprecedented attack on the freedom of the media" in Kenya.
Information Minister Mutahi Kagwe said the government remained committed to press freedom and the "promotion of responsible journalism".
Internal Security Minister John Michuki said the raids on the Standard group in Nairobi were designed to protect state security.
"If you rattle a snake, you must be prepared to be bitten by it," he said.
The BBC's Karen Allen in Nairobi says the raids are being seen as an indicator of growing political tension in the administration.
Three ministers have resigned this year after details of a corruption investigation were leaked to several newspapers.
Opposition MPs and other demonstrators marched from parliament to The Standard's offices in protest.
Opposition leader Uhuru Kenyatta described the action as a "dark day" for Kenya.
Both the president and senior opposition figure Kalonzo Musyoka have denied a report in The Standard last week that said they had been holding secret meetings.
Mask-wearing 'routine'
Police Commissioner Jasper Ombati said he had evidence that journalists were being paid to incite ethnic hatred.
He said that police officers routinely wore masks to hide their identities in sensitive cases.
Hooded men carrying AK-47 assault rifles raided the headquarters of the Standard group just after midnight.
Staff were kicked and beaten and forced to lie on the floors as offices were searched and equipment taken away, The Standard newspaper said on its website.
The printing presses were also raided. Thousands of copies of the newspaper were dragged out into the yard and set on fire.
The police denied their officers were responsible for burning the papers and promised to investigate.
Meanwhile, another group of masked men went to the offices of the Kenya Television Network.
The station was off air until 1100 GMT, and men carried away computers and transmission equipment.
(BBC)
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