Federal police, "based on the power given... by the Federal Democratic Republic of Ethiopia constitution to protect/guard federal institutions" entered Mekele "and started work", their agency said in a statement on Facebook.
"It will give guaranteed security to institutions administered by the (federal) government, including airports, electric power, telecom services, banking and other public service rendering institutions."
An agreement signed on November 2 in the South African capital Pretoria provides for the disarmament of rebel forces, the re-establishment of federal authority in Tigray and the reopening of access to the region.
The process shifted up a gear on December 22 when the two sides agreed to create a joint monitoring and compliance mechanism.
On December 26, a federal delegation made their first visit to Mekele since the peace deal was signed, bringing along the CEOs of major corporations.
On Wednesday, the national carrier Ethiopian Airlines resumed commercial flights between Addis Ababa and Mekele after an 18-month shutdown.
Aid is starting to flow to the crippled region and some essential services are being restored.
Mekele was hooked back up to the national grid on December 6, while the country's biggest bank, the Commercial Bank of Ethiopia, announced on December 19 that financial operations had resumed in some towns.
War between the federal government and Tigray began in November 2020.
Prime Minister Abiy Ahmed sent troops into the northern region after accusing its ruling party, the Tigray People's Liberation Front (TPLF), of attacking army bases.
Estimates of casualties vary widely, with the United States saying that as many as half a million people have died.
The conflict also unleashed one of the world's worst humanitarian disasters in recent times, displacing more than two million people and driving hundreds of thousands to the brink of famine.