The attack occurred at around 10am local time (9am BST). Broadcaster RTVE said a government delegation confirmed the victim suffered 10 stab wounds.
The report said 12 patrols were searching for the suspects, with underwater and aerial surveillance units also joining the search.
At 12pm on Monday (11am BST), the mayor and councillors gathered at the entrance to the Regional Palace of Toledo to observe a minute's silence in memory of Mateo and a 17-year-old girl who were shot dead in nearby Otero on Saturday.
Mateo's cousin and family spokesman Acel Sanchez told Spanish media he had no idea who was responsible.
He was quoted as saying: “He was a very good boy, very affectionate, playing with his friends, someone arrived and we don't have any more information. It's a difficult time at the moment.”
Moshon, with a population of about 5,000, has declared three days of official mourning.
“This is a sad day. The whole city is torn apart by pain, every killing is painful, but it is even more painful that an innocent child was killed this morning while playing soccer with his friends,” the city's mayor, Concepcion Cedillo, told reporters on Sunday.
Milagros Tolón, the Spanish government representative for the Castile-La Mancha region, wrote on X: “Deeply saddened to hear of the murder of a minor in #Mosejón (Toledo) after being attacked with a sharp object.
“The Civil Defense Forces launched an extensive operation to apprehend the perpetrators.
“I send all my love to the family and friends of the deceased and the people of Moseshon.”
Emiliano García-Pazu, the president of the Castile-La Mancha region, said he was shocked by the events in Mosejón and hoped the perpetrators would be found quickly.
“This situation is unacceptable and just punishment must be meted out.”
