By Victoria Cavaliere

NEWARK, New Jersey (Reuters) - Newark Mayor Cory Booker slid to an easy unofficial victory in New Jersey's special Democratic primary held on Tuesday in the race to fill an empty U.S. Senate seat that has long been in Democratic hands.

If he wins the general election in October, Booker would be the state's first black U.S. senator.

Booker, who had been widely expected to win after holding strong leads in public opinion polls, was declared the winner by the Star-Ledger newspaper on and The Bergen Record on less than an hour after polls closed. said that with 18 percent of precincts reporting, Booker claimed 55 percent of the vote, far ahead of his closest competitor, Representative Frank Pallone, who got 27 percent.

On Twitter, Booker tweeted: "Honored to receive Dem nomination for #NJSen today. This is our victory - thank you. Please continue to run with me."

On the Republican side of the ticket, Steve Lonegan, former mayor of Bogota, New Jersey, and a Tea Party conservative, was declared the unofficial winner over Alieta Eck, a physician, for the party's spot in the October 16 special general election. Lonegan claimed 80 percent, compared with 20 percent for Eck, said.

A poll earlier this month showed Booker beating Lonegan 54 to 29 percent in a general election match-up.

Concern and controversy over the timing of the election emerged promptly after the Senate seat came open with the death in June of Senator Frank Lautenberg at age 89. The liberal Democrat had been elected to the Senate five times.

To fill the seat, Republican Governor Chris Christie called the August 13 primary and set a special election for October 16, three weeks ahead of the November 5 general election, when he is seeking re-election.

Christie named a Republican to fill the seat temporarily who said he would not seek the seat on a permanent basis.

Democrats charged that the two fall elections should have been scheduled for the same day but that Christie was avoiding being on the same ballot as Booker, who could attract both strong Democratic and minority turnout.

After the polls closed on Tuesday, Booker supporters began to gather at the plaza in front of Newark's Prudential Center, where the mayor was expected to appear.

"Overall he's feeling good," said Booker's deputy campaign manager Silvia Alvarez. "The numbers are where we expected given that it was raining this morning, and it's a primary in August."

Turnout was thin, with the primary being held amid the summer vacation season.

Also seeking the Democratic Party's nomination with Pallone were Representative Rush Holt and state Assembly Speaker Sheila Oliver. But they found little traction against the well-funded and well-known Booker, considered a rising political star.

(Additional reporting by Barbara Goldberg in Maplewood, New Jersey; Writing by Ellen Wulfhorst; Editing by Eric Walsh, Dan Grebler and Lisa Shumaker)