Polling stations opened early on Monday, four years after the state upheld the controversial 2000 election result, where President George Bush won by a disputed 537 votes.
Within hours a Democrat leader had lodged a complaint regarding faulty ballot papers.
Queues are said to be more than an hour long in some areas while others faced a computer crash when the touch screen system failed.
Florida is one of 32 states where voters are allowed to cast their ballots before election day.
As the battle for votes in this state has intensified, Democrat Kerry has launched a discernible drive to stop Jewish voters straying over to the opposition.
Courting Jewish voters
In 2000, Jews voted 4-to-1 for Democrats Al Gore and Joe Lieberman, the first Jewish candidate on a major party's presidential ticket.
But President George Bush has built a reputation as a strong backer of Israel, and has courted Jewish voters in the hope that even a slight increase in support could make a difference in another tight election.
Kerry told voters on Monday in West Palm Beach that he would do a better job than Bush of "holding those Arab countries accountable for funding terrorism".
"We will do a better job than Bush of holding those Arab countries accountable for funding terrorism. We'll do a better job of protecting the state of Israel than they are today"
Democratic presidential candidate
"We'll do a better job of protecting the state of Israel than they are today," Kerry said.
Supporters held signs distributed by the campaign that read: "Jewish Americans for Kerry" and wore stickers and T-shirts that said "Kerry-Edwards" in Hebrew.
After wooing Jewish voters with a little Hebrew, Kerry tossed out some French to communicate with Haitian immigrants at a rally later in Orlando. Kerry speaks fluent French, but usually avoids doing so in public.
Appealing to Haitians
But not in Florida, where many Haitians have moved to seek relief from problems in their homeland.
"Je vais aider les Haitiens," Kerry said, promising to help the Haitian people.
The words in French were unusual, but the outreach to Jewish voters is standard in Kerry's Florida pitch.
Kerry's spokeswoman Stephanie Cutter said he wanted to reassure them that he would continue his record of fighting for Israel.
"By and large the Jewish population in Florida knows who's on their side and who isn't," she said.
Kerry's paternal grandfather was a Czech Jew who immigrated to the United States and changed his name to Kerry from Kohn in order not to be identified as a Jew.
Kerry's Jewish roots were discovered last year by the Boston Globe, but he had not mentioned it during his Florida stops.
Boston Globe exposed Kerry's
Jewish roots last year
Instead, he talked about his visits to Israel and his pro-Israeli voting record.
"I've had the privilege of flying a jet in Israel, learning firsthand how tight that security is, how close the borders are, how tiny and fragile it is," Kerry said.
"I've climbed to the top of Masada and I've stood on the top of Masada and yelled out as the Air Force recruits and others used to from the side of that cliff, the words 'Am Yisrael Chai!'"
Kerry's use of the Hebrew cry which means "The people of Israel live" implying the longevity and eternity of the Jewish people on earth, delighted the crowd.
The symbolism of Masada - the desert mountain where Jewish rebels chose suicide over capture - still looms large in Israel as soldiers come at the start of their military training to pledge allegiance to the state.
Sharyn Wachs, wearing one of the campaign's Hebrew stickers on her shirt, said Kerry seemed "really united" with Israel and she was touched by his story of climbing Masada since she had done it twice herself.
She has been angry with Bush's invasion of Iraq, and left the rally to go and cast her vote for Kerry.
"Let's just hope he can come through," she said. "That's the thing - they can say anything to get elected."
Kerry promised that he would stay engaged in the Middle East and help create a Palestinian entity with which Israel could negotiate.
Kerry has made remarks against
the Israeli wall in the past
"You don't have one today, so you have to build a fence and you have to do what you are doing," he said.
Bush-Cheney campaign spokesman Steve Schmidt said Kerry had a different position when speaking to an Arab-American gathering near Detroit one year ago.
He said the separation fence separating Israel from the Palestinian territories was a "barrier to peace".