By Nopporn Wong-Anan

BANGKOK (Reuters) - Thousands of truckers went on a half-day strike in Thailand on Wednesday demanding government help against rising fuel prices, the latest in a series of protests that have swept across Asia and Europe.

Transport groups said they were ready to block roads into Bangkok next week, piling more pressure on a government already facing a prolonged street protest over its attempt to change the constitution.

The protest has sparked fears of a military coup less than two years after the army booted out pro-business prime minister Thaksin Shinawatra.

"If the government doesn't meet our demands by June 17, we will mobilise our trucks in Bangkok," Thongyu Khongkan, secretary-general of the Land Transport Federation of Thailand, which has 400,000 trucks under its banner, told Reuters.

Specific demands included a discount of 3-baht (5 pence) per litre of diesel for six months, as well as cheap loans to convert engines to compressed natural gas, he said.

State-run Thai refineries agreed last month to sell diesel to Bangkok bus companies at an 8 percent discount. Other sectors, such as fishermen and truckers, are expected to benefit from a similar arrangement.

The soaring cost of fuel, stemming from world crude prices that touched a record $139 a barrel last week, has sparked protests around the world by consumers and transport groups.

In Europe, Spaniards have started stockpiling food for fear a truckers' strike that has disrupted deliveries could cause shortages. Traders at Madrid's main food wholesale market said supplies of fresh food would start to run out soon.

Portuguese drivers have joined the strike and there were also protests in France.

In Asia, where many countries are net crude importers, governments have been forced to cut fuel subsidies and let pump prices climb, putting a squeeze on households already struggling with higher costs of basic food such as rice.

South Korea's cabinet offered to resign in the face of huge street protests on Tuesday about the policies of its unpopular President Lee Myung-bak.

Lee said Asia's fourth-largest economy could be heading into crisis because of surging resource prices and slowing growth. Producer price inflation in the world's fifth-largest crude oil importer was near a 10-year high last month.

South Korean truck drivers voted to strike on Monday, ignoring a $10.2 billion (5.22 billion pounds) government aid package designed to cushion the impact of fuel price rises.

Malaysian Prime Minister Abdullah Ahmad Badawi on Tuesday pledged 1 billion ringgit (156.9 million pounds) in extra spending for the politically key state of Sarawak, to shore up support there among lawmakers unhappy over a jump in fuel costs.

The opposition is calling for protests later this week against the government's decision to raise petrol prices by 41 percent and diesel by 63 percent.