BEIJING, September 7, 2008 (AFP) - The United States and Britain set the early pace at the Paralympics Sunday as Olympic swimmer Natalie du Toit claimed her first gold and Chinese officials played down an embarrassing security breach.

On day one South Africa's du Toit, who finished 16th in the Olympics women's 10 kilometres swim, timed 1min 6.74sec in the 100m butterfly in her category, one of 16 golds on offer in the pool.

The 24-year-old amputee is aiming for four more wins to match her 2004 gold tally, but warned sweeping each of her five events would be difficult.

'I'm not swimming the 100m backstroke so I just have to do five races. I have to get out there and push the barriers,' she said.

'They're going to be tough. There's a 50m freestyle that's going to be very, very close but I've gone out there and I've had a really good first race so hopefully the next four will come and also be good,' she added.

Du Toit won five golds and a silver at the Athens Paralympics. She lost her left leg in a road accident in 2001, after narrowly missing qualification for the Sydney Olympics a year earlier.

Chinese organisers meanwhile said a woman who burst into the opening ceremony and tried to remove her clothes was mentally ill and dismissed suggestions she was mounting a political protest.

Press photos and TV pictures showed staff grappling with the woman, clad only in blue jeans and a bra, as she lay on the floor of the 'Bird's Nest' National Stadium during Saturday's extravaganza.

The intrusion, as 4,000 athletes were entering the stadium, was one of the worst security breaches seen during the Olympics or Paralympics.

'She tried to enter the field and remove her shirt. She was stopped by our staff,' said Zhu Jing, a press officer with the Beijing organising committee.

'It was confirmed later by authorities that she was mentally ill,' Zhu added.

The United States and Britain both finished the opening day with four golds.

Britain picked up where they left off in the Olympic cycling, grabbing three track golds. Britain and Australia notched up three world records apiece.

Britain's Darren Kenny knocked almost six seconds off his own world record in the qualifying round of the men's individual pursuit before going on to take gold.

The athletics programme starts Monday, with much attention focused on South Africa's double amputee track sensation Oscar Pistorius -- dubbed 'Blade Runner' due to the carbon fibre blades with which he has won a host of titles.

Pistorius has set his sights on a clean sweep in the sprints and is also eyeing the 400m world record.

More than 4,000 competitors from nearly 150 countries and regions are battling for 472 gold medals in 20 sports at the eye-catching venues used for last month's Olympics such as the 'Bird's Nest' and the Water Cube.

Host China, which topped the medals table at the 2004 Athens Paralympics with 63 golds ahead of Britain and Canada, is widely expected to dominate again -- and even more comprehensively than at last month's Olympics.

The 20 sports at the Paralympics, which ends on September 17, include athletics, swimming, powerlifting, wheelchair fencing and five-a-side and seven-a-side football, as well as the lesser-known goalball and boccia.

China, eager to showcase the progress it has made in catering for the disabled, has adopted the motto 'Two Games with Equal Splendour,' for the Games, comparing them to the Olympics.

Like the Olympics, the Paralympics have not been free of scandal and there will again be a focus in Beijing to stamp out any cheating.