Bells from Madrid’s 650 churches tolled for five minutes from 0737 (0637 GMT) when the first of 10 co-ordinated blasts hit commuter-packed trains.
A five-minute silence was held at midday before the opening of Madrid’s Park of the Absent, where trees have been planted to remember the victims.
Some survivors are boycotting events, complaining of political manipulation.
The explosions came three days before a general election, in which the Socialists ousted the right-wing Popular Party (PP) of then Prime Minister Jose Maria Aznar.
