The appeal came days after the Tigray People’s Liberation Front (TPLF) said it had captured two cities about 400km (250 miles) from Addis Ababa.
The federal government has declared a nationwide state of emergency.
The US has urged the rebels not to try and take Addis Ababa.
The capital has a population of more than five million.
“We oppose any TPLF move to Addis or any TPLF move to besiege Addis,” said Jeffrey Feltman, the US special envoy for the Horn of Africa.
War broke out a year ago in the northern Tigray region.
The TPLF has since advanced into the neighbouring regions of Amhara and Afar.
Thousands of people have been killed and more than two million have fled their homes since the conflict began.
The TPLF says it wants to break a siege of Tigray, where the UN estimates 400,000 people are living in “famine-like conditions”.
International efforts to broker a ceasefire between the government and the rebels, who initially fell out over Prime Minister Ahmed Abiy’s political reforms, have so far failed.
The head of Addis Ababa’s peace and security bureau gave residents two days to register weapons or to give them to the government or a relative who could use firearms, the state-linked Fana Broadcasting Corporation reported.
Kenea Yadeta added that people should “organise” themselves, and work with the security services to “control” their neighbourhoods, the report said.