According to the decree, published by the outlet, Puce dismissed Usakovs, who is the leader of the party Harmony, the largest movement representing Latvia's Russian-speaking population, from his position as chairman of Riga's local legislature.
Usakovs has served as Riga's mayor since 2009.
Over recent months, Usakovs has survived a number of no-confidence votes over his alleged links to a corruption scandal around a municipally-owned monopolist transport operator. Usakovs has repeatedly denied the accusations being levelled against him.
In March, Usakovs said he would leave his post as mayor in order to be able to represent his party in the European Parliament elections, scheduled for May.