By Jeff Zeleny, Arlette Saenz and Sunlen Miller

Prince Harry arrived in Washington on Thursday, opening an eight-day tour of the United States with a visit to the White House and Capitol Hill, where a large crowd of adoring fans were on hand to greet him.

At the Russell Senate Office Building, he spent about 30 minutes touring a landmine exhibit with Senator John McCain. He later made a surprise stop just down Pennsylvania Avenue at the White House.

"Well surprise!" First Lady Michelle Obama said, speaking at a "Joining Forces" event honoring military mothers in the East Room of the White House.

PHOTOS: Prince Harry Tours the US

This is the first trip to the United States for Prince Harry since his headline-enduing visit to Las Vegas last August. His weeklong trip, which will also take him to Colorado, New York and Connecticut, is focusing on promoting the rehabilitation of wounded American and British troops.

He caused a stir the moment his motorcade reached Capitol Hill, where hundreds of young women were waiting with their iPhone cameras for the arrival of the 28-year-old who is the third in line to the British throne.

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He toured the landmine exhibit silently, speaking only to McCain and his tour guides. He did not make public remarks, but McCain later said Harry was deeply engaged in the exhibit.

"He was a very nice young man," McCain told reporters later. "He was asking in-depth questions."

The exhibit displays the work done by the non-profit charity Halo Trust, which helps to clear landmines and hazardous debris of war in war torn countries, an organization championed by Princess Diana. Cindy McCain, the Arizona senator's wife, is a board member for the charity but was unable to attend due to knee surgery.

It was a solemn and serious tour, interrupted only by adoring cheers from his fans, nearly all of whom seemed to be taking breaks from their Congressional jobs.

"I'm sure it's not the first time he's seen that," McCain said later.

He went from the Capitol to the White House, dropping by the event to honor military mothers and spouses. He helped some of the children of service members write messages on special packages for their mothers.

"We are absolutely thrilled that he could be with us today," Mrs. Obama said. "He wanted to be here to personally thank you for your service."

He is scheduled to attend an evening reception honoring the 25th anniversary of Halo, the group dedicated to the removal of landmines, at the British ambassador's residence in Washington. On Friday, he is set to visit Arlington National Cemetery, to lay wreath at the Tomb of the Unknown Soldier in Arlington National Cemetery and Walter Reed National Military Medical Center.