Visiting US first lady Michelle Obama toured China's Great Wall on Sunday, after her arrival at the US embassy in Beijing earlier in the day sparked a minor security alert.

She and her daughters strolled along a popular section of the wall and rode down a toboggan on a smoggy afternoon, as part of a week-long visit focused on education and "soft issues" rather than politics.

Vendors at the tourist site northeast of the capital appeared to have removed T-shirts which are normally on sale showing President Obama in a Mao hat, with one merchant revealing a boxful of the souvenirs stored in the back of her tent.

Earlier in the day, as Obama's motorcade arrived at the US embassy for a roundtable with educators, parents and students, two people just outside the premises began shouting.

Police and men in gray sweatshirts and pants hurried to subdue them, and it was unclear why they were yelling.

Since arriving in the capital on Thursday night, Obama -- whose mother is also accompanying her -- has also played table tennis with students and toured the Forbidden City with her counterpart Peng Liyuan.

She briefly trod political ground in a speech Saturday morning at Peking University's Stanford Centre, calling for greater freedoms while refraining from naming China.

"As my husband has said, we respect the uniqueness of other cultures and societies," Obama told a crowd of about 200 students, most of them from the United States.

"But when it comes to expressing yourself freely, and worshipping as you choose, and having open access to information -- we believe those are universal rights that are the birthright of every person on this planet," she said.

"We believe that all people deserve the opportunity to fulfil their highest potential, as I was able to do in the United States."

But the majority of Obama's speech was devoted to encouraging American students to study in China.

She touted the "100,000 Strong" initiative announced by President Obama during his 2009 visit to Beijing. The programme aims to raise the number and the socioeconomic diversity of Americans studying in China.

After her speech, Obama held a virtual roundtable with a group of American students, then took a tour with her family of the Summer Palace, a former imperial getaway not far from Peking University.

In opening remarks at Sunday's discussion in the embassy, she stressed that "education is an important focus for me".

"It's personal, because I wouldn't be where I am today without my parents investing and pushing me to get a good education," she said.

The participants discussed standardised testing, opportunities to go to college and the treatment of ethnic minorities in the education system.

Obama will also visit the northern city of Xian, site of the famous ancient Terracotta Warriors, and Chengdu in the southwest, home to the country's iconic pandas.