But to a group of young army officers, most of them under 30 and later to be known as 'the captains of April', it was the signal that this was their D-day to move against 48 years of dark dictatorial rule. They had been tipped to go on stand-by 90 minutes earlier when the radio station played the first agreed signal, Portugal's 1974 entry in the Eurovision song contest in Brighton, 'E Depois Do Adeus' ('And after the goodbye'). The song did not fare well in Brighton but will be remembered here longer than any of the contest's winners.
Other coded messages had helped prepare the coup and fix 'H-hour' - the move from the barracks. 'Auntie Aurora will arrive at 0300 hours on the 25th. Love, cousin Antonio,' read one message between officers.
By 6pm, it was virtually all over, without a single shot fired by the army. A legendary army major, Fernando Salgueiro Maia, then only 29 and who died in 1992 of cancer, stormed into Lisbon in a convoy of armoured cars and forced the surrender of Marcello Caetano. Caetano had inherited power in 1968 from the ailing Antonio de Oliveira Salazar, who had ruled by fear for four decades.
Like Salazar, Caetano used the secret police to keep a lid on opposition. Despite years of independence wars in Africa, it had not occurred to Caetano that his biggest threat might come not from underground political groups but the military. Hardly a family had not had someone killed or wounded in Angola, Mozambique or Cape Verde. Some 9,000 young Portuguese had died and more than 25,000 had been maimed in the dictatorship's efforts to hold on to the colonies. That was the key to the captain's revolt. It was a coup that turned immediately, by force of human nature, into a revolution.
Although the new Armed Forces Movement (MFA) called on people to stay home to avoid bloodshed, they had to give vent to half a century of repression. After daylight, even before Caetano had surrendered, they poured into the streets to kiss and hug soldiers. Flowers stuffed into the barrels of rifles by young women gave 25 April 1974 the title the Carnation Revolution.
The long nightmare of dictatorship was over but a period of political insomnia had begun. There was confusion as to the MFA's aim. Was it just another coup a-la-South America? Time magazine carried a cover painting of the chief of the new junta, monocled cavalry general Antonio Spinola, looking menacing and with the heading 'Coup in Portugal', not revolution. The confusion proved justified and as the new military leaders turned out to straddle the ideological specntrum, civil war looked possible as power was increasingly up for grabs. Many Portuguese had been shocked to see the mushrooming red flags and hammer- and-sickle banners, the Soviet, Cuba and China-like and murals, some of which still give a touch of faded colour to the drab docks of Lisbon. The coup had come six days before May Day, and exiled Communist Party leader Alvaro Cunhal returned on 30 April to join hundreds of thousands in celebrationsthat passed peacefully.
Portugal lived through two years of uncertainty, with the military holding the balance of power but itself split. Their programme centred on the three Ds - democracy, development and decolonisation. By the end of 1975, the colonies had been granted independence - only Macau remains to be handed over to China in 1999.
After General Spinola, the junta chief who had resigned in an earlier split with leftist officers, failed in an abortive coup in March 1975, the junta swung to the left. Banks and big industries were nationalised but the first democratic elections, first for a constituent assembly on 25 April, 1975, then for a new legislature a year later, put an end to the pro-Moscow road. The Communists, with around 12 per cent of the vote, trailed behind the Socialist Party of Mario Soares with 35 per cent.
The coup-cum-revolution's 'second D' - democracy - had been achieved. In 1985 the Social Democratic Party of Anibal Cavaco Silva won power and has held it since. It was that same year that Portugal signed up for the European Union, representing the trigger for the 'third D' - development. Portugal receives an average of pounds 4m a day from the EU; Mr Cavaco Silva has replaced nationalisation with privatisation.
Mr Soares, now 68 and his personal popularity still intact, was elected President in 1986 and will retire when his second term ends in 1996. From that day 20 years ago, before which Portugal was doubly isolated from Europe - Franco's Spain not only hid behind the Ppyrenees but the Spanish-Portuguese frontier used to close down at night - this nation has turned from gazing out towards Africa to alliance with and reliance on Europe.
That is not to say all is peaceful. Economic crisis and unemployment have pushed the Socialists ahead in the polls. Some 'captains of April' say 'Cavaquismo' - Mr Cavaco Silva's neo-liberal policies - have sold out their revolution. 'It's not exactly the Portugal we dreamed about,' says Colonel Vitor Alves, a junior officer during the coup. Disenchantment is not confined to the left. Outside the decrepit former secret-police headquarters, near Lisbon's docks, there is a plaque to the four people killed in the coup. They were civilians shot by secret police as they stormed the building in anger. The plaque, however, has been defaced by a large black swastika.