BEIJING, Dec 29 (Reuter) - A bomb exploded outside a government office in the Tibetan capital, Lhasa, and Chinese authorities on Sunday described the blast that shattered windows for 100 metres as "an appalling act of terrorism."

The Christmas Day bomb, the largest so far set off by anti-Chinese activists in the restive Himalayan region of Tibet, resulted in no casualties but caused widespread damage, local officials said by telephone from Lhasa.

"It was a huge explosion that could be heard a long way off," said a government official, who refused to be identified.

"This was the biggest bomb blast by counterrevolutionary elements in Lhasa," a party official said. "None of the terrorists has been arrested, but one thing is certain this was done by plotters of the Dalai (Lama) clique."

The bomb exploded on the doorstep of the Lhasa City Chengguan District Government Office in the early hours of December 25, ripping apart the gateway to the buildings that house the main city government offices, the government official said.

"Windows for at least 100 metres (yards) around suffered some degree of damage," he said. "But because the explosion was in the middle of the night, no one was wounded". He declined to give further details.

"The bombing... was an organised, planned, and targeted counterrevolutionary bombing incident staged by the Dalai (Lama) clique," local radio said on Friday quoting a circular issued by the regional Communist Party committee and government.

It was "a serious counterrevolutionary political incident and an appalling act of terrorism," the circular said according to the radio report monitored by the British Broadcasting Corp and made available on Sunday.

The London-based Tibet Information Network (TIN) said five people were wounded, some seriously. They included two nightwatchmen at the office and shopkeepers living nearby.

A nearby branch of the Bank of China was also reported to have been damaged along with two hotels, TIN said.

"The act of terrorism staged by the Dalai (Lama) clique is opposed by the people of Lhasa City, as well as by people all over the world," the party broadcast said.

"It fully demonstrates that the Dalai clique has cast off its previously so-called peaceful disguise to openly oppose the people of Tibet and has reached a point when it puts up a last-ditch struggle," it said.

China regularly blames followers of Tibet's exiled god-king, the Dalai Lama, for anti-Chinese unrest that erupts sporadically in the strategic mountainous region that straddles the Himalayas and runs along China's sensitive border with India.

The Dalai Lama, who won the Nobel Peace Prize in 1989 for this non-violent campaign to win autonomy for his homeland, says he wants self-government and freedom of worship in the deeply religious Buddhist region. He fled China in 1959 after an abortive uprising against Beijing rule.

Speaking at a meeting of party officials in Lhasa on Friday, the vice chairman of the autonomous regional government, Gyamco, called for a campaign of retaliation.

"We should wage a tit-for-tat struggle against the Dalai clique's sabotage," he said.

"We should, once again, stage another campaign across Tibet to thoroughly expose and criticise the Dalai clique, heighten our alertness and strengthen preventive measures so as to keep the situation stable," the radio quoted him as saying.

Several much smaller bombs have been been set off in Lhasa over the last two years, including one in 1995 that caused slight damage to a plaque donated by Beijing and another last March outside the headquarters of the Tibet regional government.