WASHINGTON — Intent on not overlooking clues about any terrorist plots after the attacks of Sept. 11, 2001, the government spread a now-familiar slogan: “If you see something, say something.” Less visibly, it built a national database to better harness reports of suspicious activity in the hunt for terrorists.

On Thursday, five California men opened a legal front over the recurring tensions between collective security measures and individual rights by filing a lawsuit that challenges the Suspicious Activity Reporting database. They contend that it is too easy for people engaged in innocuous activities to be put into the database and scrutinized as if they were a threat.

The plaintiffs include two white photographers who were confronted by security guards at a natural gas tank and by the police at a refinery; an Egyptian-American who tried to buy a large number of computers at a Best Buy store; a Pakistani-American who was looking around in a train station with his mother, who wore a Muslim head scarf; and a white Muslim convert who was looking at a flight simulator game on the Internet.

Each contends that he was added to the database for his behavior, although only two, according to previously disclosed government documents, have been able to prove it. The lawsuit argues that federal standards are too lax in allowing a security guard’s or a police officer’s report to be uploaded into the national database.

The Justice Department did not…