(updates with more quotes, background)

BEIJING, Sept 5 (Reuter) - U.S. First Lady Hillary Rodham Clinton attacked China on Tuesday, saying women taking part in a grassroots forum near Beijing had been prohibited from fully taking part and or expressing themselves freely.

"It is indefensible that many women in non-governmental organisations, who wished to participate in this conference, have not been able to attend -- or have been prohibited from fully taking part," she told the U.N. Fourth World Conference on Women.

"Let me be clear. Freedom means the right of people to assemble, organise and debate openly," she told a plenary session.

"It means respecting the views of those who may disagree with the views of their governments," she said in a hard-hitting speech clearly directed at host China, accused of harassing participants with heavy-handed surveillance and security measures at the grassroots Non-Governmental Organisations Forum on Women.

"It means not taking citizens away from their loved ones and jailing them, mistreating them, or denying them their freedom or dignity because of the peaceful expression of their ideas and opinions," she told the world's biggest United Nations conference.

The issue of police harassment and surveillance has marred much of the grasssroots forum on women that opened last Wednesday and runs parallel to the U.N. conference.

Exiled Tibetan women delegates have been followed and harassed by security guards, radical feminists such as lesbians have had their Chinese-language publicity pamphlets confiscated by police and human rights activists have come under intense surveillance.

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.

But many groups have ignored his restriction of protests to a school sports field.

The top Chinese police official at the conference said speech was free but only if it did not contain language aimed at splitting China -- a reference to what Beijing calls attempts by Tibet's exiled Dalai Lama to win independence for the Himalayan region.

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

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