Havana (AFP) - In hundreds of schools, hospitals and public buildings, Cubans signed a "solemn oath" on Monday to defend the revolution following the death of communist leader Fidel Castro.

Instead of leaving messages in books of condolence, Cubans were invited to endorse the "concept of the revolution" defined by Castro in a speech in 2000, six years before illness forced him to hand power to his brother, Raul.

"We will keep fighting for these ideas. We swear!" says the oath to which Cubans signed their names, three days after Castro died at age 90.

"The signature shows the desire of Cubans to make this socialist revolution irreversible," said retired lieutenant colonel Rigoberto Cerolio, 80, at a school in Havana.

While Cubans lined up to sign the oath across the island, hundreds of thousands flocked to Havana's Revolution Square to pay tribute to Castro at a memorial installed inside the monument to independence hero Jose Marti.

Leonardo Guijarro used his iPad to record the memorial, which consisted of white roses flanking a picture of a young, black-bearded Fidel in military fatigues during the guerrilla war.

Guijarro may have immortalized the moment with a modern gadget and worn Adidas sneakers, but the 22-year-old university history student shared the firm belief and desire of other Castro supporters that his revolution will live on after his death.

"Fidel has died but he is still with us," he said after he filed past Castro's picture.

"There's a Fidel in every Cuban citizen," said Guijarro, who like many recalled that Castro brought free education and health care to the island after ousting dictator Fulgencio Batista in 1959.

- 'Like a father' -

Several generations of Cubans streamed past the memorial: stylish university students, veterans in military fatigues, boys and girls in school uniforms, couples pushing strollers, retirees consoling each other.

While dissidents label Castro a dictator who jailed or forced detractors into exile, for "Fidelistas" he could do no wrong.

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"He cared for Cubans like a father who cares for his children," said 52-year-old economist Norma Maria Diaz la Torre, whose husband wrapped his arm around her shoulder as she cried after viewing the portrait.

"Everything he did was for the good of the people," she said.

Her spouse, 58-year-old engineer Leonardo Sarria, said Castro's death will change nothing.

"If we give up one iota, we will no longer be Cuba," he said.

Luis Rene Allouis, a 22-year-old sociology student with a crow tattooed on his chest, said he was ready to defend the system against any coup attempt.

"I would take an AK-47 and go to the mountains, like Fidel," Allouis said, recalling Castro's guerrilla war against Batista's regime.

- 'Just struggle' -

Some from the older generation said Castro liberated Cuba from a terrible era under Batista.

"Abject poverty, lack of opportunities, prostitution: That was Cuba. It was a disastrous society," said Angel Ierselo, an 80-year-old writer.

"We are convinced that Fidel's struggle was a just one, incredibly just, and we will follow the same path," Ierselo said.

Augustin Fivale Hernandez, 80, and his wife, Elsy Vanela, held hands with teary eyes as they left the memorial.

Fivale said they were blue-collar workers before the revolution but "everything changed" after 1959.

He was a messenger in Cuban television and his salary went up.

"I was nobody. I knew nothing. With Fidel I was able to study. I retired as a unit boss in television," Fivale said.

Like the government, the couple blames the country's economic troubles on the decades-old US embargo.

"(Fidel) made so many great promises and fulfilled them. If he didn't fulfill them it was the blockade's fault," Vanela said.

- No mistakes -

While President Raul Castro has implemented modest economic reforms, Fidelistas see them as an extension of the revolution and are confident that he will continue his older brother's legacy.

"Raul doesn't have the same stature but he has learned. The other one (Fidel) was more energetic, he is smoother, but deep down they're the same," said Alicia Rodriguez, 66, a retiree who worked in the distribution of medicine.

Jose Carlos Perez, a 19-year-old studying to teach English, was disappointed that Castro did not lie in state or that the urn holding his ashes was not put on display.

"I wanted to see him because he is the country's most important figure," he said.

Among Fidelistas, nobody could name a single mistake by their icon.

Said 67-year-old retired seamstress Alba Rodrigo: "I don't think he has done anything bad."