NEW DELHI — Factories and workshops across India were up and running today after major electrical grid collapses caused the world’s two worst power blackouts.

An estimated 620 million people lost state-provided electricity when India’s northern, eastern and northeastern grids failed Tuesday afternoon. It followed Monday’s failure of the northern grid, which left 370 million people powerless.

Utility workers struggled throughout the day Tuesday to return power to the 20 affected states, restoring most of the system within hours of the failure. India’s new Power Minister Veerappa Moily told reporters that by thismorning power had been fully restored.

Moily, who took over the top power ministry position Tuesday, said an investigation had begun and he did not want to point fingers or speculate about the cause.

Other officials said the blackout might have been the result of states drawing too much power from the grid. Some analysts dismissed that explanation, saying if that was the cause, such collapses would happen all the time.

Moily warned states to stick to a disciplined withdrawal of power.