China is sensitive about political anniversaries and in Hong Kong - now part of China, but a part where political protest is permitted - the administration is as jumpy as its new masters. So Hong Kong will permit what is likely to be a large rally commemorating the massacre, but will deny entry to people like Mr Xu and Mr Zhang, as well as better known dissidents like Wang Dan and Wei Jingsheng.
Szeto Wah, one of Hong Kong's leading pro-democracy campaigners, said the ban on the entry of the dissidents, "shows the authorities suppressing the freedom of speech which has been enjoyed by Hong Kong". Martin Lee, the leader of the Democratic Party, said the ban raised "grave concerns" about Hong Kong's independence.
The administration has tried to take the political heat out of its decision, insisting that it was a routine matter decided under existing regulations. "All government decisions are being taken in the long-term interest of Hong Kong," said Stephen Lam, the government's chief spokesman.
There is little doubt, however, that the decision will be welcomed in Peking, where concern is mounting about what may happen not only on the Tiananmen anniversary but also during celebrations in October to mark half a century of Communist rule.
Two liberal-minded intellectual magazines have been shut in anticipation of the anniversaries, and employees of the government, Communist Party and affiliated organisations have been threatened with dismissal if they make public statements which could be interpreted as supporting democracy.
"Sensitive" places like universities have been targeted for special surveillance, according to dissident sources in Hong Kong, and a propaganda campaign emphasising the dangers of "instability" is being stepped up.
The beleaguered China Democracy Party knows that large-scale protests will only provoke a sharp response, so it has urged supporters to engage in low-key protests such as lighting candles.
Back in Hong Kong the administration is treading a careful line between not antagonising Peking and trying to assuage the groundswell of support for the democracy movement. Despite its reputation for being apolitical, Hong Kong regularly marks the Tiananmen anniversary with large rallies. The organisers of the this year's expect that the ban on dissidents from elsewhere in China will increase local interest.