From AFP:
Around 20 people who planned to attend the year-end dinner of the Independent Chinese PEN society, a local branch of an international association of writers, have been barred from attending, its chairman Liu Xiaobo told AFP.
Some have been put under house arrest while others were warned by police to stay at home, he said. Some of the people put under surveillance were not even members of the association.
Police summoned Liu on Friday to tell him to call off the annual dinner, which was to be attended by up to 50 intellectuals from across China, he said. [Full text]
See also an AP report, which says two writers were detained as part of the crackdown.