By Tom Ramstack and Joshua Roberts

WASHINGTON (Reuters) - A man shot himself dead in front of the U.S. Capitol on Saturday, police said, sparking a temporary security lockdown at the complex on one of the busiest days for tourists in Washington.

The man, who was wearing a backpack and had carried a rolling suitcase and a sign to the site, fired a single shot at himself, Capitol Police Chief Kim Dine told reporters.

Dine said the man's sign had a "social justice" message but he did not immediately provide more details and did not release the man's name.

Authorities said they placed the Capitol on a lockdown as a precaution. A bomb squad technician examined the suitcase and the backpack and officials later gave the all-clear, lifting the lockdown after about two hours.

"There seems to be no nexus to terrorism or anything related to that," Dine said.

The incident occurred as Congress was out of session and few lawmakers were in town. But the city was crowded with tourists visiting for the popular annual Cherry Blossom festival.

The disturbance blocked traffic but did not appear to disrupt most visitors to Washington.

Robert Bishop, a real estate developer from Annapolis, Maryland, who was visiting Washington and was at the scene of the shooting, said the man who killed himself had a sign that he thinks said, "Tax the 1 percent."

The deadline for Americans to submit their tax returns is Wednesday.

The man shot himself next to a fountain between the Capitol building and a street. About 50 people were in the area at the time, Bishop said.

"Everybody started ducking and started to run," he said.

Near the White House, about 2 miles (3 km) east of the Capitol, authorities moved tourists away from the grounds on Saturday afternoon as a precaution before later reopening the area, a law enforcement official said.

Lawmakers are due to return to work on Monday.

The shooting is the latest in a string of security incidents at high-profile buildings in the nation's capital.

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In 2013, police shot and killed a woman after she rammed security barricades with her car near the White House before racing toward the Capitol. Also in 2013, a government contractor opened fire at the Washington Navy Yard, killing 12 people.

More recently, a man pleaded guilty to charges of running into the White House in September armed with a knife before being tackled, a security breach that helped lead to a shake-up in the U.S. Secret Service.

(Reporting by Sandra Maler, Susan Heavey, Yeganeh Torbati, Roberta Rampton, Eric Walsh in Washington and Alex Dobuzinskis in Los Angeles,; Editing by Frances Kerry and Marguerita Choy)