According to reports in his televised announcement, Jammeh decried “extraordinary” foreign interference in his country's politics.
He also stated that “any acts of disobedience to the laws The Gambia, incitement of violence and acts intended to disturb public order and peace” are banned under the State of Emergency.
Jammeh said security forces had been instructed to “maintain absolute peace, law and order”.
Earlier, the National Assembly passed a motion condemning what it called the “unlawful and malicious interference” of the African Union and the country's neighbour, Senegal, in The Gambia's affairs.
The state of emergency follows the Economic Community of West African States (ECOWAS) warning, for Jammeh to step down by Wednesday midnight or be removed forcefully.