The tens of millions of voters faced new election laws and, in many places, new voting machines - aimed at remedying problems that produced a bitterly disputed outcome four years ago.

But election officials remained worried that some of the changes could instead ensure a repeat controversy in a race as tightly contested as the one between George W Bush and John Kerry.

An army of lawyers from both parties manned polls in battleground states, and the legal wrangling over ballot issues continued to rage hours before the polls opened. In Ohio, Republican moves for an aggressive programme challenging voters at the polls was upheld by a federal appellate court.

In other battleground states, Democrats challenged the challengers. Democratic officials in Philadelphia, for example, threatened to file federal lawsuits against individual poll challengers who violated citizens' voting rights through harassment or intimidation.

Election officials also fretted about provisional ballots - used by people whose names did not appear on voter rolls. Such ballots could not be counted until after election day and, for the first time, were being mandated nationwide.

Given these and other problems, election directors in battleground states prepared for long lines, legal challenges and glitches that could leave the outcome in dispute for days or weeks.

``There will be several states where we will not know the winner on election night,'' predicted New Mexico Secretary of State Rebecca Vigil-Giron.

Jan Baran, a Republican election lawyer not involved in this year's campaign, said: ``If it's close, we'll be right back to where we were four years ago.''

Extra security was also in place, amid terrorism fears. In Lake County, Ohio, law officers were guarding voting machines with the same vigilance ``as they do the nuclear power plant'', election director Jan Clair said.THE WASHINGTON POST