A spokeswoman for the Kensington Palace told CNN that it was "aware of the incident" but wouldn't comment on security matters.
The young royal, who started school at school last week, apparently likes playing with popular children's building blocks, Lego. 'It looked as if the police were trying to break up something and get between people'.
She remains in custody after being arrested on Wednesday.
Chalon-sur-Saône : deux femmes attaquées au marteau, l'agresseur en fuite
Le parquet demande à ne pas diffuser, sur les réseaux sociaux, de messages faisant état du positionnement des forces de l'ordre. Les gendarmes sont en soutien de la police et ont dépêché un hélicoptère sur place, selon nos informations.
Wife Kate Middleton - who recently confirmed her third pregnancy - was also expected to join the pair on the youngster's big day, but was forced to pull out after coming down with severe morning sickness.
Residents near Thomas' School in Battersea, west London, reportedly saw police vehicles arrive to arrest the suspect. And can we talk about how precious his little blue uniform was?
London police are part of the protective security arrangements for the Prince.
The school has 560 boys and girls aged from four to 13, with around 20 pupils in each class. Fees are £17,604 a year, and increase to £19,884 a year for those in year three and above.
A typical day might start with a midmorning snack of fruit or a fresh baked pain aux raisin, followed by a lunch of salmon filet with a salsa verde or smoked BBQ chicken fajitas and a Portuguese egg tart with fresh strawberries or a fresh baked oatmeal raisin cookie with a banana milkshake for dessert.