Undercover New York police officers traveled around the United States and to Europe to observe activists who planned to protest at the 2004 Republican National Convention — including hundreds who showed no sign of illegal intent, a newspaper reported.

Posing as activists or sympathizers, the officers attended meetings of political groups in at least 15 U.S. states and filed reports with the police department's intelligence division, The New York Times reported on its Web site Saturday.

The officers involved in the "RNC Intelligence Squad" then identified certain groups as potential threats, the Times reported, citing hundreds of still-secret reports it viewed from the police department's intelligence division. The police often shared information with departments in other cities.

Police spokesman Paul Browne said the activities were legal. He said the operation was an essential part of preparations for the huge crowds that came to the city during the convention.

"Detectives collected information both in-state and out-of-state to learn in advance what was coming our way," Browne told the Times.

The secret digests said some of the groups planned acts such as blocking intersections and hacking into Web sites. But the Times reported that the vast majority of the reports it viewed described people who gave no obvious sign of wrongdoing, such as members of the satirical performance-art group "Billionaires for Bush" and a group that had planned concerts with political speeches.

Donna Lieberman, the executive director of the New York Civil Liberties Union, said the revelations of "spying" were shocking.

"The New York Civil Liberties Union condemns this operation and is considering legal action against the police department," she said in a statement.

More than 1,800 people were arrested at the four-day convention at Madison Square Garden, where President George W. Bush accepted his party's nomination for a second term in office. The convention was policed by as many as 10,000 officers.

Pending civil rights lawsuits have challenged the legitimacy of the arrests. Documents released under a court order in January showed that arrested protesters were held before their initial court appearances for up to six times longer than those arrested on charges unrelated to the convention.

The documents also show that the 2001 terrorist attacks heavily influenced the city's decision to detain and fingerprint hundreds of protesters at the convention.

In 2003, a federal judge — at the behest of Mayor Michael Bloomberg — broadened the NYPD's authority to investigate political, social and religious groups. Browne insisted police have not abused the new guidelines.

Browne told the Times that the 18 months of preparation before the convention allowed hundreds of thousands of people to demonstrate while also ensuring that the convention had relatively few disruptions.

"It was a great success, and despite provocations, such as demonstrators throwing faux feces in the faces of police officers, the NYPD showed professionalism and restraint," he told the Times.