Speaking to Radio Mocambique President Nyusi said he saw bodies floating int he water, villages had disappeared and two rivers had burst their banks.
"With every hour and day that passes, our worst fears become increasingly real", he said in a statement "Many drowned while others were killed in their sleep from swift and unexpected rockfalls which demolished their homes".
Nyusi spoke after flying over the port city of Beira and viewing the flooding and devastation.
Up to 90% of Beira, Mozambique's fourth largest city, was reportedly destroyed. The cyclone knocked out electricity, shut down the airport and cut off access to the city by road. Tens of thousands of people, primarily in impoverished rural areas, have been cut off from roads and telephones.
Cyclone Idai barreled across Mozambique, Malawi and Zimbabwe with flash floods and ferocious winds, killing people and destroying homes.
Cyclone Idai killed at least 157 people in Zimbabwe and Mozambique as it tore across Southern Africa, officials and state media said on Monday, and vast areas of land have been flooded, roads destroyed and communications disrupted.
"Massive and horrifying" destruction in the Mozambican city of Beira, authorities and the Red Cross said on March 18, 2019.
"The situation is awful". "The scale of devastation is enormous", said LeSueur.
The Beira airport has been shutdown since the storm struck.
While the physical impact of Idai is beginning to emerge, the human impact is unclear.
The charity's Jamie LeSueur said: 'Almost everything is destroyed.
Communication is hard in some areas and anxious relatives are trying to reach their loved ones through the less damaged roads, but increased traffic is now slowing down relief efforts.
Mozambique's environment Minister Celso Correia told news agency AFP said the deaths occurred in the hard-hit coastal community of Beira and nearby inland Dondo district.
'Beira has been severely battered.
"The biggest threat we have now, even bigger than the cyclone, is floods", warned the governor of Sofala province, Alberto Mondlane.
The United Nations said that more than 100 people have died in weeks of heavy rain and flooding in Mozambique and Malawi, where villages were left underwater.
The Ministry of Information said Mnangagwa had cut short his trip to the UAE to attend to the humanitarian crisis developing in eastern Zimbabwe, but the President faced criticism over his decision to travel in the first place having seen the devastating damage the cyclone had already done in Mozambique and Malawi.
Today (Monday) Cyclone Idai has tracked a deadly path across Zimbabwe, battering the eastern and southern parts of the country, according a government official.