Beijing has warned Germany of "defending Tibet independence," saying the match host must ensure respect between the countries. The activists said they only wanted to highlight the plight of Tibetans in China.
On Saturday, Chinese U-20 players ran off the field in the 25th minute after six activists waved Tibetan flags during the game.
"We are firmly opposed to any country or any individual offering support to separatist, anti-China and terrorist activities or activities defending Tibet independence, in any form or under any pretext," said China's Foreign Ministry spokesman, Lu Kang, during a press briefing.
"I must stress that mutual respect is what the official host should provide their guest, and that respect between any two countries should be mutual."
Ronny Zimmerman, vice president of the German Football Association (DFB), suggested that China's response was unmerited.
"We cannot ban the protests, there is the right to freedom of expression here and certain rules apply," said Zimmerman. "As a guest, you should be able to handle it calmly and stand above such actions."
Activists from the Tibet-Initiative Germany, which organized the protest at the match in Mainz, said they wanted to highlight the plight of Tibetans.
"We want to draw attention to the unlawful and violent occupation of Tibet and the suppression of fundamental human rights," one of the activists told the German Press Agency (dpa).
China's communist forces "peacefully liberated" Tibet in 1950, according to Beijing. The communist government has since insisted that Tibetans enjoy extensive freedoms, and claimed responsibility for the region's economic growth.
Beijing has long considered the Dalai Lama a "separatist." The Dalai Lama fled to India after a failed uprising in 1959, although he is still widely revered by Tibetans in China.