WASHINGTON (REUTERS) - US Secret Service Director Julia Pierson resigned under fire on Wednesday after a series of security lapses came to light involving the protection for President Barack Obama.

Homeland Security Secretary Jeh Johnson announced Pierson's resignation and said Joseph Clancy, a veteran of the agency, was named acting director.

Pierson, who had been in her position since March 2013, faced growing calls from lawmakers to step down in the fallout from a Sept. 19 incident in which an Iraq war veteran clambered over the White House fence, sprinted across the lawn and got deep inside the mansion before he was stopped.

Pierson told a congressional committee on Tuesday she took "full responsibility" for security lapses.

White House spokesman Josh Earnest said Obama had called Pierson to express his appreciation to her for her long years of service.

Earnest said Obama believed Pierson's resignation was in the best interest of the agency, and that "recent and accumulating" reports of security lapses led the president to conclude new leadership was required at the Secret Service.

The development came a day after the disclosure that in a violation of protocol, an armed private security contractor with a criminal record rode on an elevator with Obama in Atlanta earlier this month and took pictures and video of the president on his phone.

The lapses, along with a 2011 incident in which seven gunshots were fired at the White House, had raised concerns across Washington that Obama was not as protected as he should be in an age of global tumult.

While all presidents have faced death threats, Obama, as the first African-American president, is believed to have received more than his predecessors.

When she was appointed the first female director of the Secret Service, Pierson was given the mission of cleaning up the agency's culture after a 2012 presidential trip to Colombia in which up to a dozen agents were found to have hired prostitutes.

An episode last March did not help the reputation of the 30-year Secret Service veteran. Three agents responsible for protecting Obama in Amsterdam were sent home and put on administration leave after what was described as a night of drinking.

Representative Darrell Issa, who led a 3-1/2-hour grilling of Pierson before the House Oversight and Government Reform Committee, said he was doubtful one resignation would produce the kind of change needed.

"Problems at the Secret Service pre-date Ms Pierson's tenure as director, and her resignation certainly does not resolve them," he said.