BEIJING (CNN) -- Chinese Premier Zhu Rongji, with his wife in tow, paid an unusual late-night call Wednesday to visiting British Prime Minister Tony Blair, which diplomats took as a sign of a burgeoning personal relationship between the two leaders.

"He invited himself over for a schmooze," said a British official of the visit. The two leaders and their wives sat together and drank tea from glass mugs.

Earlier in the day, however, the mood was not quite as cozy. Chinese authorities briefly detained Xu Wenli, an outspoken pro-democracy activist, after he gave interviews to the British media covering Blair's five-day visit to China.

After learning of Xu's detention, Blair raised the issue during a scheduled 80-minute visit with President Jiang Zemin. Xu was released a short time later.

In a speech to the British business community later in the day, Blair made the most forceful statement so far during his trip about human rights issues in China.

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"There is no disguising that we continue to have serious concerns and differences about human rights, about political and religious freedoms and about the situation in Tibet, which I discussed at length with (Jiang)," Blair said.

While crediting the Chinese communist regime for making strides against poverty, the British leader said "much remains to be done on civil and political rights."

However, Blair defended the less confrontational stance that Britain, other European nations and the United States have taken on China's human rights record. He said this new "spirit of cooperation" is helpful in discussions with the Chinese.

"When incidents like the questioning of a dissident this morning do occur, there is at least a process of dialogue in which they can be addressed and resolved," he said.

Blair and his wife, Cherie, stroll through a Chinese market

Blair goes to Shanghai Thursday

Blair was scheduled to leave Beijing Thursday for Shanghai. His visit to China is the first by a British prime minister since Britain handed Hong Kong over to China last year.

Discussions between the British and the Chinese have involved a host of joint initiatives -- increased trade, closer military contacts and cooperation on environmental protection and legal education.

Blair and his Chinese counterparts also discussed the global financial crisis, efforts to fight international terrorism and Hong Kong's progress under Chinese rule.

On Wednesday evening, Chinese state television broadcast an interview with Blair, which was recorded Tuesday in the Forbidden City. The interview, dubbed in Chinese, aired with only minor editing. A comment by Blair that human rights was a subject of differences between China and Britain was left in the broadcast.

Upon Blair's arrival Tuesday, Zhu announced that Jiang had accepted an invitation from Queen Elizabeth II to become the first Chinese head of state to visit Britain since full diplomatic relations were established between the two countries in 1972. The trip is expected to take place in the latter half of next year.

Correspondent Robert Stern and Reuters contributed to this report.