A spokesman for Taiwan's president, Ma Ying-jeou, quickly condemned the attack.

"The presidential office expresses regrets over the incident and condemns the violence stirred up by a small number of people,''Wang Yu-chi told reporters in Taipei.

Ma, who was elected in March promising a new era of relations with Beijing, has encouraged a rapid warming of business and political ties with the mainland.

China's official Xinhua news agency said Beijing had complained to Taiwan authorities over the incident.

'Astonished'

The agency said the Association for Relations Across the Taiwan Strait had written to its Taiwan counterpart to express its "strong indignation and severe condemnation" of the incident.

"We are astonished at this," it cited the letter as saying, adding that Taiwan must guarantee Zhang's safety and make sure there are no repeats.

Taiwan split from the mainland in 1949 after nationalist Kuomintang forces retreated to the island following the victory of Mao Zedong's communists in the Chinese civil war.

Beijing continues to claim the island as part of its territory and has threatened to invade if it ever moves toward formal independence.

Tuesday's assault comes several weeks before a planned visit by Zhang's immediate superior, Chen Yun-lin, considered Beijing's key point man in pushing for unity across the Taiwan Strait.