Nato forces in Afghanistan have hailed as a success the first phase of a major operation to oust the Taliban from two key districts of Helmand in the south.
More than 15,000 US, UK and Afghan troops swept into Marjah and Nad Ali before dawn. The UK defence ministry said key objectives had been met.
Two Nato deaths, including one Briton, have been confirmed. Afghan forces said 20 militants had been killed.
Operation Moshtarak is the biggest attack since the Taliban fell in 2001.
'Minimal interference'
Moshtarak - which means "together" in the local Dari language - is being led by 4,000 US Marines, supported by 4,000 British troops, with Canadians, Danes and Estonians.
The offensive began with waves of helicopters ferrying US Marines into Marjah.
British troops then flew into Nad Ali district, to the north, followed by tanks and combat units.
More than 1,000 British troops took part in trying to secure the Chah-e Anjir Triangle north-east of Marjah.
Maj-Gen Gordon Messenger told a briefing in London there had been "sporadic fighting" and the Taliban were unable to "put up a coherent response".
He said: "The key objectives have been secured and have been done so with minimal interference."
However, one British soldier, from the 1st Battalion Grenadier Guards, was killed by an explosion in Nad Ali.
Gen Messenger said that "low numbers" of insurgents had been killed during the attack.
Prime Minister Gordon Brown praised the UK forces for their action.
He said: "This day will be long remembered as the day when a new phase of the campaign to win the support of the people of Afghanistan was initiated. And I'm very proud of the exceptional role that British forces have played in that."
An Reuters correspondent in Marjah said US Marines engaged militants who were firing rockets and there was still a firefight in the area after several hours.
Lt Col Brian Christmas told the Associated Press there were sustained gun battles in four parts of Marjah.
AP reported Marjah residents as saying Taliban insurgents had fallen back into the centre of the town.
A Taliban spokesman, Qari Yousef Ahmadi, spoke to a number of news agencies, saying insurgents were still resisting in Marjah and were engaged in hit-and-run tactics.
Another Taliban spokesman, Mullah Mohammed, had earlier told ABC News that his men were pulling back to spare any civilian casualties.
Mohammad Zazai, commander of Afghan troops in the operation, said that 20 militants had been killed and 11 detained.
It was estimated there were between 400 and 1,000 militants based in the region before Operation Moshtarak was launched.
Nato's aim is to secure Marjah - a town of 80,000 - and surrounding areas as soon as possible and then bring in aid and public services.
'Civilian support'
In Kabul, Nato civilian representative Mark Sedwill said the news of the Nato-led attack "appeared to be positive" although he stressed it was still early.
He said it was vital to bring in "civilian support from the Afghan government" as soon as possible
Afghan Defence Minister Gen Rahim Wardak also said it was important to bring in local security forces quickly.
He said there had been "sporadic resistance" and there was a threat from booby-traps left by the Taliban.
"The area has been heavily mined, that's why we are moving so slowly," he said in Kabul.
One canal bridge into Marjah was particularly heavily mined.
US Marine ordnance units advanced through the town exploding bombs when located.
One other Nato fatality - from small-arms fire - in Operation Moshtarak was confirmed on Saturday, although no other details were given.
Three US soldiers were also killed by an improvised explosive device in southern Afghanistan, Nato said, although was unclear if it was related to Moshtarak.
Marjah has also long been regarded as a linchpin of the lucrative network for smuggling opium - the raw ingredient used to make heroin - harvested from Helmand's poppy fields.
Nato had distributed leaflets in the area warning of the planned offensive in a bid to limit civilian casualties.
The operation is part of an effort to secure a 320-km (200-mile) horseshoe-shaped string of towns that runs along the Helmand River, through Kandahar and on to the Pakistani border.
