Not since 2012, under the conservative government of Mariano Rajoy, had Spain held a general strike.
Transport and other public services across Spain faced serious disruption on Wednesday in the first general strike for 14 years.
The main unions called the stoppage over housing and the high cost of living.
The action came less than three weeks before parliamentary elections, and the scale of support was widely expected to be read as a measure of public discontent.
Power usage fell by 7.5% during the early hours of Wednesday, according to data from operator Red Eléctrica.
During the last general strike 14 years ago, equivalent consumption was down by more than 15%.
Demand for electricity, including from industry, is treated as one of the more reliable indicators of how widely a strike is observed.
Sánchez told Spanish television this week that workers' right to stop work should not be in doubt."What is nonsense is to question the rights of workers to strike,"he said.
Among those taking part was Gonzalo Álvarez, 27, who was visiting picket lines in central Madrid.
Rental prices in the capital have soared in recent years.
One couple pay €1,000 between them, which means each spends about half of their salary on housing, even before other bills.
Housing costs amount to nearly half the average gross salary, well above the 30% level of affordability often recommended by housing experts and public institutions.
Demonstrations were planned later on Wednesday, with the biggest protests expected in Madrid and Barcelona.