Cyclone Kenneth has made landfall in northern Mozambique, a country still recovering from another huge storm.
The cyclone has brought 220km/h (140mph) winds and has already killed three people on the island nation of Comoros.
Mozambique's National Institute of Disaster Management (INGC) said 30,000 people had been evacuated from areas likely to be hit.
Last month, Cyclone Idai caused hundreds of deaths in the region. More than 1,000 people died when the storm brought devastation to Mozambique, Malawi and Zimbabwe.
At least three million people were left in need of humanitarian assistance.
CATEGORY 4 HURRICANE
Kenneth made landfall on the north coast of Mozambique on Thursday evening, with wind speeds equivalent to a Category 4 hurricane.
There is no previous record of hurricane-force systems ever hitting the region so far north before, BBC Weather reports.
It is expected to weaken as it moves inland.
A "compulsory evacuation" of families is underway and will continue "until we have all people in secure ground", INGC spokesman Paulo Tomas said.
More than 680,000 people are said to be at risk from the cyclone, Mozambican officials said on Wednesday. Flights have already been cancelled and schools closed.
Forecasters warn it is likely to be slow-moving, meaning heavy rain is expected to fall on the area for several days.
The UN says over 600mm (23.6 inches) of rain may come - nearly double the 10-day accumulated rainfall that caused flooding in the port city of Beira, further south, during Cyclone Idai.
Forecasters at Meteo-France warned that Kenneth could trigger waves up five metres (16 feet) higher than usual off the country's northeastern shores.
"I was quite preoccupied by the sea... the wind was very strong, and I've never seen anything like it in my 15 years in Pemba," a Portuguese owner of a lodge on Wimby beach, Anabela Moreira, told AFP.
The UN's World Food Programme said it was working on an "emergency preparedness plan" with the Mozambican government and other humanitarian groups.
Comoros is still reeling from damage caused by the cyclone, which battered the islands with high winds and heavy rains. The winds have caused widespread power outages and damage to homes.
In some southern areas of neighbouring Tanzania, authorities have ordered schools and businesses to close.
Residents in the southern Tanzanian town of Mtwara were earlier told to seek higher ground and shelter, but that warning has been stood down.
The path of the cyclone has shifted meaning a major catastrophe is no longer expected, regional commissioner Gelasius Byakanwa told reporters.
"That has informed [our decision] to allow the residents to go back home and carry on with their daily activities." he said.
The two main islands in Comoros saw power outages and trees downed in high winds, Reuters news agency reports.
The International Federation of Red Cross and Red Crescent Societies shared images of the damage on social media.
In a tweet, the group confirmed it had volunteers on the ground assisting communities.
Despite Zimbabwe being further inland, officials there say they are also putting their disaster management agencies on alert.
"Drawing lessons from Cyclone Idai we cannot take chances any more," said Department of Civil Protection director Nathan Nkomo.