ISTANBUL (AFP) - Rail passengers using Turkey's new tunnel under the Bosphorus had to walk part of their journey when an electricity failure briefly halted services, just a day after its grand opening.

The 13.6-kilometre undersea tunnel in Istanbul - the world's first linking two continents - was inaugurated with great fanfare on Tuesday as the government's "project of the century".

But a power cut on Wednesday forced passengers to leave the train and walk on the tracks, the Dogan news agency said, before the problem was fixed and services resumed within a few minutes.

The tunnel, the fulfilment of a sultan's dream of 150 years ago, is part of a three-billion euro (S$5.1 billion) transport project in Turkey's biggest city.

Prime Minister Recep Tayyip Erdogan, a former mayor of Istanbul, revived the tunnel plan in 2004 as one of his mega projects for the bustling city of 16 million people - which also include a third airport, a third bridge across the Bosphorus and a canal parallel to the international waterway to ease traffic.

But his grand urban plans were one cause of the massive anti-government protests that swept the country in June, with local residents complaining they were forcing people from their homes and destroying green space.

Transport is a major problem in Istanbul, and each day two million people cross the Bosphorus via two usually jammed bridges.