HAVANA (AFP) - Hundreds of thousands of Cubans swarmed Havana's iconic Revolution Square in a tearful and nostalgic tribute to Fidel Castro on Monday, kicking off a week-long farewell to the divisive Cold War icon, as US airlines resumed direct flights to country's capital on Monday in a sign of the changing times.

Long lines of mourners entered the towering monument to independence hero Jose Marti, filing past a black-and-white picture of "El Comandante" as a young, black-bearded revolutionary carrying a rifle.

Many walked by silently while clutching flowers, some took pictures with their phones and others sobbed uncontrollably as they looked up at the portrait flanked by white roses.

But his ashes were not put on display, surprising many who had hoped to see the urn holding the remains of their hero.

The memorial began with a salvo of 21 cannon shots from a colonial fort overlooking Havana harbour.

Lourdes Rivera, a 66-year-old retired civil servant, sat on a curb and cried as she waited for her turn to enter the monument.

"He's the father of all Cubans. My dad was my dad, but he couldn't give me what (Castro) gave me. He gave me everything. My freedom. My dignity," she said.

For 36-year-old university professor Pedro Alvarez, "we know that our comandante has become immortal." Castro, whose 1959 revolution toppled a dictatorship with the promise of bringing justice and equality to the Caribbean island, was a towering figure of the 20th century.

While some saw him as a socialist hero who brought education and free health care, others labelled him a "tyrant" who caused economic hardship and sparked an exodus of Cubans seeking a better life.

Even as the ceremonies were beginning, an American Airlines flight touched down at Jose Marti international airport after setting off from Miami with a water cannon salute. It was the first time in more than 50 years a US airline provided regular commercial service to Havana.

"It's something historic. We were expecting this for a long time here," said American passenger Edgar Salveani on arrival.

The first JetBlue Airways flight from New York to Havana arrived shortly after.

Commercial flights between Cuba and the US had already resumed in August under the historic rapprochement the old Cold War foes announced in December 2014.

But these are the first to Havana.

In an added dose of symbolism, they come just three days after Castro's death at age 90 turned a page on the revolutionary leader's divisive legacy.

As the planes touched down, hundreds of thousands of Cubans were streaming onto Havana's iconic Revolution Square to pay their last respects to Castro, who ruled the island with an iron fist from 1959 to 2006.

"It's a once in a lifetime experience, it will be interesting to see how people are responding to his passing," passenger Priva Rhat told reporters on the American Airlines flight.

Air travel between the United States and Cuba had been restricted to charter flights from 1979 until earlier this year.

Direct commercial flights began on August 31, linking several US airports with nine Cuban cities, many of them in or near tourism hotspots.

There are now 110 scheduled daily flights from the United States to Cuba, 20 of them to Havana.

The direct flights were one of several watershed changes initiated after US President Barack Obama and Castro's successor, his younger brother Raul, announced a thaw after more than 50 years of hostility.

Diplomatic relations were restored in July 2015.

Washington still bans Americans from visiting Cuba as tourists, but travel is permitted for 12 other categories, including cultural and educational exchange.

In the first sign of changing times, US President Barack Obama visited Revolution Square during his historic visit to Havana in March, when he became the first US leader since 1928 to step foot in Cuba, a nation of 11 million people.

In 2014, Fidel's brother and successor, Raul Castro, announced a diplomatic detente with Obama, who has lifted some trade barriers. On Monday, the first regular flights from the United States to the Cuban capital in half a century resumed.

But US President-elect Donald Trump renewed a threat to end the thaw unless Havana makes concessions on human rights and opening up its economy.

"If Cuba is unwilling to make a better deal for the Cuban people, the Cuban/American people and the US as a whole, I will terminate deal," he said on Twitter.

Manuel Rodriguez Oliva, a 73-year-old interior ministry retiree who paid tribute to Castro, said Trump is "paranoid and crazy".

"He can break relations. We have lived without them and we will keep living."

Raul Castro has enacted gradual economic reforms. But he has firmly resisted any changes to the communist island's political system.

Government opponents hope Fidel's death will enable bolder reforms.

Fidel handed power to Raul in 2006 after undergoing emergency intestinal surgery. His cause of death on Friday at age 90 has not been disclosed.

Castro, who came to power as a cigar-chomping 32-year-old in combat fatigues, survived more than 600 assassination attempts, according to aides, as well as the failed 1961 US-backed Bay of Pigs invasion and the 1962 Cuban missile crisis.

Dissidents who were repressed by Castro's regime for years said they were happy that the "dictator" had died, but called off regular demonstrations on Sunday out of deference to those in mourning.

"We're going to remain calm, even if (Castro) is the main culprit for the misery and lack of political rights in Cuba," said veteran dissident Jose Daniel Ferrer.

Once the nine-day period of national mourning is over, "we will continue fighting the system he created," he said.

In Miami, where so many Cubans flocked in the past decades to escape Castro's policies, Cuban-Americans celebrated his death with street parties throughout the weekend.

After two days of commemorations in Havana, Castro's ashes will go on a four-day island-wide procession starting Wednesday.

They will be buried on Sunday in the southeastern city of Santiago de Cuba, the heartland of Castro's uprising against US-backed dictator Fulgencio Batista.

Fidel could do no wrong for the mourners at the Revolution Square, where Castro would often rail against the US "empire" during his legendary, marathon speeches.

Many were dressed in state uniforms - school children, soldiers, veterans, doctors and customs officers.

For them, the country's economic problems stem from the decades-old US embargo.

"If we didn't develop more, it's the fault of imperialism," said Augustin Fivale Hernandez, 80, with tearful eyes and holding his wife's hand after seeing the memorial.