SEOUL, Oct 31 (AFP) - South Korean Prime Minister Han Duck-soo on Monday promised a thorough investigation into the Halloween crush over the weekend that killed more than 150 mostly young people in the capital, plunging the country into mourning.

Officials said 154 had been killed and 149 injured, with 33 people in serious condition.

Tens of thousands of partiers had crowded into narrow streets and alleyways of Seoul's popular Itaewon district on Saturday for the first virtually unrestricted Halloween festivities in three years. Many of the revelers were in their teens and dressed in costume.

But chaos erupted when people poured into one particularly narrow and sloping alley, even after it was already packed, witnesses said.

"The government will undertake a thorough investigation into what caused this accident and do its best to make necessary institutional changes so that such an accident is not repeated," Han said as government officials met on the disaster.

"Identification has been completed for all of the 154 deceased except one, and I believe it is time for follow-up measures such as funeral procedures to be carried out in earnest," Han said. "We will do our best to provide necessary support by reflecting the opinions of the bereaved families as much as possible."

President Yoon Suk-yeol, who has declared a period of national mourning and designated Itaewon a disaster zone, visited a memorial altar near the Seoul city hall and paid his respects to victims on Monday, his office said.