New Jersey Gov. Chris Christie, several former governors, former Secretary of State Hillary Rodham Clinton and other mourners were at the service at the Park Avenue Synagogue.
Lautenberg, a liberal Democrat from New Jersey, died Monday after suffering complications from viral pneumonia. At 89, he was the oldest member of the Senate and the last of 115 World War II veterans to serve there.
Clinton, Vice President Joe Biden, U.S. Sen. Robert Menendez of New Jersey and members of Lautenberg's family were set to deliver eulogies at the service, the program for which featured a Lautenberg quote inscribed on a plaque on the exterior of the Frank R. Lautenberg Post Office and Courthouse in Newark, N.J.: "The true measure of a democracy is its dispensation of justice."
Dozens of dignitaries were designated as honorary pallbearers, including 17 senators — mostly Democrats, among them Senate Majority Leader Harry Reid, but also a few Republicans, including Sens. Richard Shelby of Alabama and Marco Rubio of Florida.
A color guard ceremony was also scheduled for Wednesday afternoon at the Frank Lautenberg Rail Station in Secaucus, N.J., where his casket was to be put on an Amtrak train to Washington.
Lautenberg was an ardent defender of Amtrak and worked to secure hundreds of millions of dollars for mass transit projects.
His casket was set to arrive at the U.S. Capitol on Thursday and lie in repose in the Senate chamber, on the Lincoln Catafalque, a bier that was built for the coffin of Abraham Lincoln.
Lautenberg, who served in the Army Signal Corps during World War II, will be buried Friday at Arlington National Cemetery.
A multimillionaire businessman, he was first elected to the Senate in 1982 and went on to serve nearly 30 years there in two stints.
He won his last race in 2008 at age 84, becoming the first New Jersey politician ever elected to five Senate terms. He had announced earlier this year he would not seek another term in 2014.
Early in his political career, he was a driving force behind the laws that banned smoking on most U.S. flights and made 21 the drinking age in all 50 states.
Copyright 2013 The Associated Press.