The multinational force is being led by the UK
Around 60 Royal Marine commandos ringed the interior ministry in the capital Kabul as Hamid Karzai was sworn in as Afghanistan's new leader.
The main aim was to provide a tight security shield for the ceremony, in which ex-president Burhanuddin Rabbani formerly handed over power watched by about 2,000 Afghan leaders and foreign diplomats.
But establishing a British presence on the ground, representing the new stabilisation force, was another part of the operation, an official at the British embassy in Kabul told BBC News Online.
"We've got to win over the Afghan population to the role we are playing, that we're here to assist them, and British forces are very good at doing that," he said.
"They're professional soldiers but they're also approachable - and forging good relations with the local community is key to any operation of this nature."
Embassy ceremony
The security role they played at the swearing-in ceremony went well, the official added.
The British soldiers were also present at a flag-raising ceremony for the re-opened embassy and later carried out foot patrols on the city streets.
Around 100 Royal Marines are already in the county to lead a force that will eventually number 3,000 to 5,000.
The force's spokesman, Major Guy Richardson, has said the troops had been deployed to help, not dictate, and the current situation in the war-ravaged country was calm.
But he added that the force could up the scale of its approach if needed.
The British troops will shoulder much of the burden of peacekeeping activities.
A United Nations Security Council vote on Thursday gave the go-ahead for the stabilisation force to operate out of the Afghan capital Kabul.
In one of their first jobs on Friday, some marines escorted convoys of foreign diplomats on the hour-long drive from Bagram airbase to Kabul.
'Fairer future'
The multinational force will be under overall US military authority, with the promise of American assistance in an emergency.
Germany said on Friday it would contribute up to 1,200 soldiers to the force.
British Prime Minister Tony Blair said that UK forces were already making a difference on the ground and had an essential role to play in stabilising the future for Afghanistan.
But he was at pains to stress that the international community wanted to assist rather than dictate.
"We will support them for the long term but it is their country, it is their future and our role is to help the people in Afghanistan to build a fairer and better future there - that future controlled and governed by the people there."
Use of force
The UN resolution also calls on the victorious Northern Alliance to withdraw its troops from Kabul, in accordance with the agreement reached last month at a multi-ethnic conference in Bonn, Germany.
But Afghanistan's Interim Defence Minister Mohammed Fahim said the troops would have no authority to disarm belligerents, interfere in Afghan affairs or use force.
Meanwhile, there were reports a man claiming to be British was being held in Pakistan on suspicion of being an al-Qaeda member.
The man is in hospital in Peshawar following his arrest after crossing the border from eastern Afghanistan.
A Foreign Office spokesman said: "At first he said he was French but it then emerged he might be British. We are looking into the situation."