The North ordered flags outside official buildings to be flown at half-mast as a mark of respect for Mr Castro, who died on Friday at the age of 90.
In the Pyongyang subway, commuters in one station crowded around a glass case containing a obituary to Mr Castro published by the ruling party newspaper, Rodong Sinmun.
Alongside a picture framed with a black border and showing a head-and-shoulder shot of a bearded Mr Castro in full military dress, the obit recalled his visit to North Korea in 1986 when he met founder-leader Kim Il-sung.
It noted the Cuban leader was awarded the title of “DPRK [North Korea] Hero” for his efforts to strengthen relations between two countries “fighting in the outposts of the anti-US, anti-imperialist struggle”.
Kim Jong-un sent a wreath to the Cuban Embassy, calling Mr Castro a “close friend and comrade” of the Korean people.
An official delegation led by senior aide and vice chairman of the ruling Workers’ Party Central Committee, Choe Ryong-hae, left for Havana on Monday to attend memorial events.
According to a Japanese agency which monitors North Korean media, Mr Castro is the first foreign political figure to be honoured in such a manner since Palestinian leader Yasser Arafat, who died in 2004.
Besides flying flags at half-mast, it was not immediately clear what the mourning period, which ends on Wednesday, would entail.
Shortly after receiving news of Mr Castro’s death, Kim Yong-nam, head of the North’s parliament, and Premier Pak Pong-ju sent a message of condolence to Mr Castro’s brother Raul, who assumed power after Fidel became too weak to continue as leader in 2008.
In it, they said that although Mr Castro has died, “the feats he performed for the Cuban revolution and the fraternal relations of friendship between the two countries would remain forever”.
Inside the daily life in North Korea
Inside the daily life in North Korea
People reading a newspaper at the metro station
Thoughts of the leaders on the tram. They have about a dozen of these on every tram, all with different thoughts
Young people training for a big upcoming festival
People at the Pyongyang's annual marathon
Many stars on one of the trolleys in Pyongyang
An intimidating poster in a primary school in North Korea.
Solar panels installed on a street lamp.
A poster on the window next to one of the venues we visited in Pyongyang
Kids playing football next to the Arch of Triumph. After a while tourists were allowed to join, so some of us did
Class in an educational center in Pyongyang (where people over 17 years old can attend any classes they choose after school, for free)
People waving at me during the Pyongyang marathon
People having a great time dancing at a public park
A metro driver in a metro station in Pyongyang
Fireworks to mark the birthday of the Eternal President Kim Il Sung on our last night in Pyongyang
My wonderful tour guide at a public park
One of the parks in Pyongyang
A person rowing some boats for the day at a river in Pyongyang
The National War Museum
Public park in Pyongyang
Because of their common hostility toward the US and similar authoritarian power structures, Cuba and North Korea had maintained very close diplomatic ties throughout the years.
The two countries established ties in 1960 and Mr Castro visited the North in 1986 to meet with Kim Il-sung, the country’s founder and Kim Jong-un’s grandfather.
Cuba became one of the few countries willing to flout international sanctions imposed on the North over its nuclear weapons programme.
In 2013, Panama seized a North Korean ship carrying an undeclared Cuban arms shipment of Soviet-era weapons and fighter jets hidden under sacks of sugar.
North Korea insisted the weapons were being shipped for repair, prior to their return.
Such fraternal sentiment toward Havana and Raul Castro, however, appears to have dimmed in Pyongyang amid a thawing of relations between Cuba and the US, who agreed to normalise ties in 2014.
Additional reporting by agencies