Protests: It began early on Saturday afternoon when people began to receive phone calls, text messages and e-mails from friends and acquaintances.
The message told them to assemble outside the Popular Party (PP) headquarters at 6 p.m. and ended with the words: "Pass on this message".
Thousands did.
As six o'clock approached, a few hundred people had arrived.They shouted, chanted and waved their fists at the PP building, protesting at the government's apparent manipulation of information about Thursday's atrocity.
An hour later the crowd had swollen to around 1,000 and, as the message spread, it grew to 3,000 people.
Many carried placards simply saying: "Peace". Others read: "Your war, our dead."
Although the demonstration was not authorised, police stood by, merely preventing the crowd getting too close to the glass facade of the PP headquarters. There were no political leaders, just anonymous citizens expressing their frustration.
Shortly before midnight, the crowd moved to the city centre at the Puerta del Sol, where they joined the several hundred people who had been holding a vigil outside the regional government buildings.
The chants grew louder, and so did the crowds. Many brought saucepans, bin lids and tin cans, and anything else they could bang together to make noise.
The protest continued there until nearly 3 a.m. yesterday morning, when many of the protesters moved to Atocha Station where they held a five-minute silence to remember those who had died there two days before.
Similar protests took place across the country. No one knew who organised them, but the massive support brought back memories of the huge anti-war demonstrations in which millions of Spaniards showed their opposition to the invasion of Iraq.
There were over 10,000 in Barcelona and thousands more in virtually every major town and city in Spain.
One of the protesters' main criticisms of the government has been its blatant manipulation of information and even downright lying.