SAN DIEGO, April 6 (Xinhua) -- The San Diego Zoo in the U.S. state of California held a special ceremony on Saturday to kick off a three-week farewell event for two giant pandas.

Twenty-seven-year-old female giant panda Bai Yun and her son, six-year-old Xiao Liwu, will leave the zoo in late April and be sent back to China, as the zoo's conservation loan agreement with China has ended.

According to the zoo, their staff are working with their colleagues in China on future panda conservation and research.

The zoo's giant panda program is the first joint research program established between China and the United States as early as in 1994, said China's Consul General in Los Angeles Zhang Ping.

"Like other joint giant panda research and conservation programs in the United States, this program has not only promoted the friendship and understanding between the Chinese and American people, but also pushed forward the bilateral collaboration and joint research on protection and conservation of endangered species of wildlife and biodiversity," he said.

Erica Kohler, director of operations of the San Diego Zoo, told Xinhua it is sad to see the two pandas leaving for China. However, it is also a time to celebrate the successful collaboration with China on the joint research of panda breeding and wild life protection.

Dubbing the pandas "the friendship ambassadors" from China, she said Bai Yun and Xiao Liwu attracted about 2 million visitors to the zoo each year.

As the first panda on loan, Bai Yun, arrived at the San Diego Zoo in 1996. The cuddly panda soon became an iconic image of the zoo and one of the most popular animals. The name of her son, Xiao Liwu, means Little Gift in English.

Gaylene Thomas, animal care supervisor of the San Diego Zoo, told Xinhua that they learned a lot from the giant pandas and gathered data from them.

Thomas said as a great mother of six baby pandas, Bai Yun has a very good character, is very easy-going.

"We are very proud to have the opportunity to be with them for the past 20-odd years," she said.

Currently, the zoo is busy preparing for the pandas' travel back to China and trying its best to make them feel comfortable during the trip, Thomas added.

Many zoo visitors lined up to say goodbye to the pandas, and wrote down their best wishes on cards and posted them on a friendship wall set up in front of the panda exhibition halls.

The farewell celebration will run through April 27.

The San Diego Zoo is globally recognized and a San Diego icon by hosting more than 4 million guests each year.