Prosecutors said the plantation manager, Malaysian national Phuah Chuan Hun, and his employee Widiantoro paid two men between 2009 and 2010 to kill the primates.

The plantation employees and the two killers, Imam Muhtarom and Mujianto, were charged with killing endangered species and all face five years in jail.

"The two men were paid one million rupiah ($111) for each orangutan and 200,000 rupiah ($22) for other monkeys," prosecutor Suroto told the Tenggarong district court.

The plantation, in East Kalimantan province on Indonesian Borneo, is a subsidiary of the publicly listed Malaysian-owned Metro Kajang Holdings.

"The two used a 4.5-millimetre calibre airsoft gun to shoot the orangutans out of trees before their six hunting dogs chased them," Suroto said.

They would then hit the orangutans afterwards with rocks or wooden sticks before tying them up with rope to take photographs as evidence, he said.

Police arrested the four men in November after photos of them with their prey, including long-nosed monkeys found only on Borneo, circulated around the community.

The men were charged with killing one baby and two adult orangutans, but police said earlier that at least 20 had been killed based on receipts of from the company amounting to 25 million rupiah ($2,775).

Experts say there are about 50,000 to 60,000 orangutans left in the wild, 80 percent of them in Indonesia and the rest in Malaysia.

They are faced with extinction from poaching and the rapid destruction of their forest habitat, driven largely by palm oil and paper plantations.

The trial will resume next week.

A worker in charge of the enclosure at Jinan Wildlife World in Shandong province was at lunch when the incident happened last Saturday and it took officials 10 minutes to open a gate so the tourists could escape, reports said.

None of the 27 tourists on the bus were injured, but the driver's hand was hurt when the tigers jumped on the vehicle and broke the windscreen, the official Xinhua news agency said.

Terrified visitors hid under their seats as a group of up to eight tigers bit the vehicle's tyres, destroyed its windshield wipers and broke windows, the Global Times reported.

Attempts to call police failed because there was no mobile phone reception, it added.

"We are deeply sorry for the accident," Wu Yanfei, deputy manager of Jinan Wildlife World, was quoted by Xinhua as saying.

The park has launched an investigation into the attack and compensated the tourists, Xinhua said.

China says it has nearly 6,000 endangered tigers in captivity, but just 50 to 60 living in the wild in its northeast.

In the 1980s, China set up tiger farms to try to preserve the big cats, intending to release some into the wild.

But the farms have come under the international spotlight, with some conservation groups saying they use the cats for their body parts, while media reports have exposed poor conditions at zoos and animal parks.

A number of attacks on humans by captive tigers have been reported in recent years.

Last year, a tour bus driver was mauled to death by a Siberian tiger at a breeding centre in the northeastern province of Heilongjiang after he got out of his vehicle to check on a mechanical problem.

In 2010, a zoo keeper was killed by a Bengal tiger at Shanghai Zoo after apparently forgetting to lock the animal's cage.