Six trains carrying 18,300 people came to a halt during morning rush hour at stations near Saitama north of Tokyo, said an official of JR East, which operates the line.

Television footage showed hundreds of passengers carefully walking on the tracks in the rain after getting out of the trains, guided by rail company officials.

At least 45 passengers complained of illness because they had stayed in the crowded trains for up to four and a half hours without air conditioning, a fire department spokesman said.

"Of them 27 passengers were sent to hospital and one of them was in a slightly serious condition," the spokesman said.

The JR East official said the blackout was caused by a cut in overhead wires on the rails near Saitama station and that the firm had launched an investigation.