For 12 hours, they questioned him. They brought him water, but no food. And they debated the document that had led him here: Charter 08, a call for sweeping political change in China.
It's gotten to be an old story here: A clutch of activists challenges the government; the government jails one or two to scare others into silence.
But the movement around Charter 08 is different, say human rights groups and Mr. Zhang, who helped draft the document.
A month after its release, Charter 08 is still making waves in China. A wide cross-section of citizens has expressed support online. And the government, nervous about social unrest and the approaching anniversary of Tiananmen Square, has contacted – and in some cases, interrogated and threatened – at least dozens of the manifesto's original signers.
"This text is having a lot of impact – people are debating and signing it online," says Nicholas Bequelin, China researcher for Human Rights Watch. "This is a landmark in terms of its appeal, and [the] attention that it has provoked."
Charter 08 calls for an end to one-party authoritarian rule and lays out a vision for a rights-based society – an electoral democracy, under the rule of law, with equality for peasants and city-dwellers and protected freedoms of speech and expression.
Similar calls have been made before; all have failed to weaken the Chinese Communist Party's grip on power. But activists say this manifesto is significant in several respects.
First, thousands of citizens of all backgrounds – peasants, teenage netizens, prominent lawyers, former party members – have added their names to the petition, not just the usual gadflies. They reflect a minority unwilling to accept the party's vision for China.
Second, the Internet has vastly expanded the charter's reach, with no central organization. That makes it a new kind of threat to a government concerned about organized challenges to its rule.
"It's a testament to the power of the Internet," says Joshua Rosenzweig, of the Dui Hua Foundation, a group that promotes human rights in China. "[It's] allowed Charter 08 to galvanize and bring together a lot of people from different walks of life and locations."
Meanwhile, the government has gone after key players behind the document. Liu Xiaobo, a coauthor of Charter 08, was detained on Dec. 8, the eve of the charter's scheduled publication online. He is being held by authorities at a Beijing hotel, according to Human Rights Watch.
The group has called Mr. Liu's detention "the most significant Chinese dissident case in a decade." "He was seen as being pretty untouchable," says Mr. Bequelin. "The fact he was taken away shatters that notion, and indicates an escalation in the repression of independent thought in China."
Zhang was also arrested on Dec. 8, but later released. Less then three weeks after the pair's detention, sitting in a private back room of a Beijing coffeeshop, he explained the appeal of the document he helped craft. "I think Charter 08 articulates what many Chinese people want to say," he says.
The direct inspiration is Czech activists' call for freedom in 1977, during the days of Soviet occupation. Charter 08's critique is blunt: "The political reality, which is plain for anyone to see, is that China has many laws but no rule of law; it has a constitution but no constitutional government. The ruling elite continues to cling to its authoritarian power and fights off any move toward political change."
Zhang says more than 300,000 websites now link to the charter, and it's being discussed on blogs, QQ (a popular Chinese instant message service and website) groups, and other chat rooms. "It's impossible to block information in society now," he says.
One user posted the following on the Independent Review, an online forum: "The CCP cannot even accept such peaceful and rational suggestions? I will sign the charter!"
Zhang says police seized from his home four computers, books, documents, DVDs, and all of his, his wife's, and their parents' cash and credit cards. Just hours after the Monitor interviewed him on Dec. 26, Zhang was detained again, according to the group China Human Rights Defenders.
"His interrogators sternly warned Mr. Zhang about 'severe consequences' to his family and friends if he continued to give media interviews or engage in any other activities promoting Charter 08," the group wrote in a press release.
Zhang and other activists say the government's reaction to the document reflects its worries ahead of the 20th anniversary of the 1989 Tiananmen Square crackdown, and as China's economic engine begins to sputter.
Beijing has banned state-run media interviews with charter signers, banned articles by charter signers, and ordered a crackdown on journalists who signed the charter, according to Radio Free Asia. Websites publishing Charter 08 have been blocked, though it's easily found using a proxy server.
Zhang says he expected this reaction to Charter 08 and is "mentally prepared" for jail. He notes police have treated him well so far – due to his party background, he guesses.
"Sent down" to Sichuan Province during the Cultural Revolution to make missile parts in cave factories, Zhang later became a high-ranking party youth league official – only to be stripped of his post in 1989 after he spoke out in support of protesters. Now he's vulnerable to charges of "inciting subversion" for his role in Charter 08.
Zhang says his home is watched around the clock by at least two men, whom he brings hot water and magazines. "We get along very well. We're all humans, they're only doing their job," says Zhang. "We're not enemies."
"I don't want to be jailed, but I have no choice," he continues. "We have to stand up and fight for democracy."
• Zhang Yajun contributed to this story.