By Lynne O'Donnell

HUAIROU, China, Sep 4 (Reuter) - A British minister asked China not to intervene on U.N. sites, South Africa's Winnie Mandela was caught in a scuffle with Chinese security staff and 150 women broke a police line, embarrassing Beijing as it hosted a U.N. women's conference.

The issue of police harassment and surveillance that has marred the parallel Non-Governmental Organizations (NGO) Forum on Women that opened last Wednesday, haunted China's gala launch of the United Nations Fourth World Conference on Women Monday.

"While we accept that China has a different way of life, this is a U.N. conference, not a Chinese conference. If they invite the U.N. here they should keep to U.N. rules," Baroness Chalker, leading Britain's delegation to the conference, told reporters.

Britain's minister of overseas development cited people being followed at the NGO forum and said that while this was not commonplace "the fact that it is happening at all is obviously against the U.N. regulations for such a conference."

Winnie Mandela, estranged wife of South African President Nelson Mandela, missed the welcome extravaganza when she and 20 South African delegates turned up late at the Great Hall of the People and tried unsuccessfully to force a way inside.

Delegates formed a cordon around Mandela protecting her from the worst of the pushing and shoving with Chinese security guards in the melee that lasted several minutes.

The fracas, involving such a high-profile personality, came as Chinese authorities were dampening criticism of earlier incidents of heavy-handed security at the NGO forum.

At the U.N.-endorsed forum, 150 women holding a protest against women's suffering, defied Chinese police and broke through a cordon marking the boundary of the site.

The women, vanguard of a 1,000-strong demonstration of women clad in black and carrying candles, defied a Chinese order to confine their demonstrations to the site.

"Stop, stop," the police said, holding up their hands.

"Let's go, let's go," the women responded. Walking forward led by Thai groups, they crossed the "red line" demarcated by Chinese police.

"We shall overcome," the women sang.

A second line of young policewomen stopped the marchers. The Asian Women's Human Rights Group staged their Women in Black demonstration to protest against violence and discrimination against women.

Carrying placards and banners attacking sex-trafficking, rape in armed conflicts and abuse of Tibetan women, the marchers dispersed in the face of the second police line and an enormous security net, including a police blimp overhead.

"Don't forget Tiananmen Square," "Bosnia is bleeding," "Poverty has a woman's voice too," "Brides are not for burning," read some of their placards.

Chinese Vice Minister of Public Security Tian Qiyu sought last week to restrict activities at the NGO forum, saying demonstrations would be allowed only in a designated area and China would not tolerate slander or attacks on its leaders.

Many groups have ignored his restriction of confining protests to a middle-school sports field.

U.N. officials say China has no right to order or enforce such restrictions inside the forum and the venue of the U.N. conference that opened Monday.

"They are not letting us go out of this gate for any action," said Dr Ranjana Kumar of India. "Is this a jail?"

Disputes over the intense security at the forum have threatened to overshadow the gathering of 25,000 grassroots women whose aim is to focus on the battle for equality for women and their rights.