"As a father and grandfather, the death of several innocent children is particularly horrifying and must be a wake-up call to all sides to refrain from violence, exercise restraint, and respect the rule of law," the top American diplomat was quoted as saying on Friday.
At least 23 people have been killed, among them four children, and nearly 800 others injured since anti-government protests broke out in November in the South-East Asian nation.
The demonstrators are trying to force out Prime Minister Yingluck Shinawatra and her government.
"I call upon Thai authorities to investigate these attacks swiftly and bring those responsible to justice," Kerry said.
"Violence is not an acceptable means of resolving political differences,” he added.
Stating that Washington does not take sides in Thai politics, Kerry voiced concern about ‘the employment of other tactics that undermine Thailand's democratic values and processes, inhibit compromise and further exacerbate political tensions’.