Vice President Joseph R. Biden presided over a cheerful swearing-in ceremony in the Senate chamber, where spectators in a full gallery peered down from above and applauded.
In multiple blocs, the senators — 13 of them for the first time — filed to the front of the chamber and raised their right hands to take the oath of office from Mr. Biden.
The smiling vice president congratulated each of them before they signed the Senate Oath Book.
Senate Majority Leader Harry Reid, Nevada Democrat, called the meeting to order and asked for the first quorum call of the 113th Congress.
Mr. Reid said there may be one roll call vote later in the day, but added, “I hope that’s not the case.” He then asked people in the rustling chamber and galleries to be quiet, joking all the noise was coming “from the Democrat side.”
Sen. Patrick Leahy, Vermont Democrat, repeatedly asked the chamber to be in order.
Minutes prior to the swearing-in ceremony, Sen. Mark Kirk made a dramatic return to the Capitol after suffering a stroke almost a year ago. The Illinois Republican, assisted by Mr. Biden and Sen. Joe Manchin III, West Virginia Democrat, slowly climbed the 45 steps outside the building leading to the second-floor Senate chamber. His fellow senators gathered to cheer him on, erupting in applause when he began and finished his climb.