Xinhua said four severely injured people had been taken to the provincial capital, Kunming , for advanced care, while the other 11 were being treated in Qiaojia. Citing local authorities, Xinhua said the bomb was set off at about 9am in the community office in the town of Baihetan.

Xinhua quoted a Kunming newspaper as saying the woman set off explosives she had taken into the demolition bureau inside the community office after she was asked to sign a house-relocation agreement.

The bomber and two staff of the bureau died on the spot. The identity of the fourth person who died is not yet known. Pictures posted online by mainland news portals showed debris scattered outside the office, including part of a limb, and a body on the floor.

Qiaojia township said the authorities were investigating the cause of the blast and the victims' families were 'emotionally stable'. It refused to admit the blast was caused by a suicide bomber, and vowed to lock up the perpetrator. It had established a taskforce to lead the investigation and maintain social stability.

A man protesting at a forced demolition in Qiaojia was killed two weeks ago, with residents blaming official brutality. Local media said Ding Fachao, from Laodian village, was taken to the township office on April 17 but was dead the next day.

On Wednesday a woman jumped to her death in Guangzhou's Yangji village in a protest against the forced demolition of her home.

Late last year, months of protests over a government land grab in the village of Wukan in Guangdong led to the removal of the village committee after a protest leader died in custody.

A year ago, three explosions were set off outside government buildings in Fuzhou , Jiangxi , by a man whose house was demolished to make way for an expressway. Two people were killed and six injured.

News of yesterday's tragedy has been widely circulated and discussed online, with many internet users expressing doubts about official statements and saluting the suicide bomber, calling her a 'heroine' and a 'pioneer defending rights'.

One microblog user said: 'When laws can no longer extend justice, this behaviour is the most righteous.'

Another microblogger asked: 'How can the emotions of the victims' family members be stable only five hours after the blast, with human flesh still scattered across the floor?'

Tang, the Guangzhou rights lawyer, who has long represented people protesting at forced demolitions and requisitioning of land, said: 'It is common to see people committing suicide in protests over forced evictions, but the suicide bomber in Yunnan represents a shift of public attitudes.

'Conflicts such as this have been accumulating for a long time and have risen to a new level. In the past, many would give in to threats, pressure and repeated harassment from local authorities, with the aid of the police force and thugs.

'But some who are completely hopeless have decided there is no other way out than to die together with those who hurt them.'

Tang doubted whether it would lead to a fundamental change in the system to respect civil rights and property rights.

Zhan Jiang, professor of communications at Beijing Foreign Studies University, said a woman suicide bomber was unheard of in China and the tragedy would make the authorities take a serious look at the problems caused by forced demolitions.

However, he said more violent and aggressive protests would continue across the nation. 'To Chinese, home and land are everything. They will fight to keep the last bit of land they have, at any cost,' Zhan said.