Two protesters were arrested, but not before they forced the ship to stop, causing acute embarrassment to defence officials and to Prime Minister John Howard who had just farewelled HMAS Sydney with its 230-strong crew on their way to war in Iraq.

News footage showed the protester, one of a group, climbing from a small boat up the hull of the guided-missile frigate, presumably after throwing a rope with attached grappling hook on to the bow rail to secure himself, and then hanging from the bow.

The incident occurred despite intensive security surrounding the ship at the farewell ceremony at a Sydney naval base attended also by Defence Minister Robert Hill and defence force chief, General Peter Cosgrove.

Security was stepped up around landmarks and key infrastructure after a similar incident in which two anti-war protesters painted "NO WAR" on the top of the Sydney Opera House on the day the war started last month.

Clinging to the bow of the ship, the protester unfurled the "No-War" banner as police craft buzzed the vessel.

Another protester, apparently from Greenpeace, then climbed from a small runabout up the stern of the warship before he was tackled by police, who appeared to cut him free.

Earlier, police swooped on another anti-war protester as he tried a paddle a surfboard towards the warship.

The Sydney was pulling away from a naval base wharf when an inflatable craft with a "no-war" banner approached.

TV footage showed a protester leaping from the inflatable on to a surfboard but he was almost immediately stopped by two police divers who jumped into the water from another boat.

The Sydney pulled to a halt as water police moved in to remove the protester hanging from the bow of the ship. Police said they had arrested two protesters.

Howard, a staunch supporter of the US campaign to oust the Iraqi regime, earlier thanked the Sydney's crew for their service and said it was a proud moment for the navy as the ship prepared to leave Sydney Harbour.

"It is my hope that during your time, in the not too distant future, the really hostile part of the war to disarm Iraq will have terminated," Howard told sailors and their families.

"But that does not mean there will not be very important work following the formal cessation of hostilities."

Defence spokesman Brigadier Mike Hannan said later that security at the farewell was mainly a matter for the New South Wales state police but the Defence Force supported the right of people to protest against the war.

"Close-in security of our forces, force protection, is an important matter for us," Hannan told reporters.

"But obviously the force protection measures that the navy takes in Sydney Harbour are very different from those it would take in the Persian Gulf. Obviously security on the harbour is a police matter for the most part.

"Our big concern, however, is the safety of the protesters.

"The ADF supports the right of Australians to protest. It's an important part of our democracy, but this is a dangerous thing to do and we really think we'd discourage it from that point of view."

SPACE.WIRE