Hundreds of protesters have staged a rally and march ahead of the US Democratic Party convention in Boston.
Protesters are angry that they will be confined to a fenced-off zone
Short-lived scuffles broke out among a few protesters - some of whom have radically different agendas - as they gathered on Boston Common.
The route of the march itself was tightly controlled by security forces, who have deployed warplanes and harbour patrols amid new threats of attack.
The four-day convention will see John Kerry anointed presidential candidate.
Tens of thousands of people have been arriving in Boston to attend the party meeting, which starts on Monday.
The confrontations took place as protesters gathered on Boston Common ahead of the march organised by the International Answer anti-war coalition, reports BBC News Online's Kevin Anderson at the scene.
A man wearing neo-Nazi-themed clothes was thrown out by members of the generally liberal crowd. Anti-abortion activists were also escorted from the park by police after arguing with other protesters.
Demonstrators marched a short distance to a fenced-off "free speech zone" designated by police concerned about security threats to delegates.
The zone, which is more than a block from the convention centre, is surrounded by concrete blocks, chain-link fencing and razor-wire.
Protesters have previously complained that it was unsafe and denied them their First Amendment right to free speech.
They had hoped the city would be forced to tear it down, but their hopes were dashed on Thursday when Judge Douglas Woodlock ruled - despite some reluctance, he said - that the zone should stay.
Correspondents say the measures surrounding the gathering of 5,000 delegates, 15,000 guests and 15,000 journalists are unprecedented.
In addition to extra air, land and sea patrols, about 40 miles (65km) of roads are being closed for the duration of the convention and some subway stops are also being shut down.
Security fears have seen air, sea and land patrols increased
Officials are concerned that al-Qaeda may be planning another massive attack inside the US to disrupt the elections.
Separately, the FBI is warning that a radical domestic group may try to disrupt the convention, possibly by attacking media vehicles with bombs.
Delegates are all to be brought to the venue by bus and will have to pass through security fences.
Mr Kerry is expected to arrive in his hometown of Boston on Wednesday, before accepting his party's nomination to run for the White House at the climax of the convention on Thursday.
Before then, he and his pick for vice-president, John Edwards, are continuing to campaign elsewhere in the country.
Mr Kerry told a crowd in Sioux City, Iowa: "John Edwards and I are determined that we are going to be champions for the middle class, the folks who built this country.
"We go to Boston, to the birthplace of the revolution of America and the possibilities of the future. And from there we go to the White House."
The Democrat team will challenge incumbents George W Bush and Dick Cheney in the 2 November poll.