The site is only open as far up as the frescoes

Buddhist monks had lit lamps and incense sticks to pacify the wasps, believed to have been disturbed by children throwing stones at a nest.

Dozens of tourists were taken to hospital with swollen limbs.

Hundreds of thousands of visitors each year visit the World Heritage Site, 150km north-east of Colombo.

Vesak festival

The wasps are still active in the area and visitors will be restricted to the lower site featuring the 1,500-year-old frescoes of bare-chested Sigiriya maidens.

There are about a dozen nests at Sigiriya but Thilak Dunuwilla, the government's chief research officer in the region, said the scale of this attack was unprecedented.

"We are hoping they will settle down soon so that we can reopen the fortress fully," he told Associated Press.

The 5th century fortress is very busy at this time of year - the Vesak festival that marks Buddha's birth, death and enlightenment.

Officials at the site, also known as Lion Rock, said a few workers were stung on Monday but the swarm had calmed thanks to the offerings and improved weather.

Higher winds meant more wasps were settling in their hives.

Sri Lanka's Island newspaper thanked the wasps for trying to keep down tourist numbers, which it said were a major threat to heritage sites.

It said the wasps were the reincarnation of the troops of King Kasyapa who built the fortress in the year 475.