Tens of thousands of trade unionists, including those from a group linked to India's ruling party, marched through the streets of the capital on Wednesday to protest food prices, piling pressure on a government already under fire over graft.

The demonstration in New Delhi was the latest in a wave of protests sweeping across the world, including the Middle East and Africa, ignited by a worldwide spike in food prices.

India, Asia's third-largest economy and home to more than a billion people, has been grappling with double-digit food inflation. Hundreds of millions of poor have been hit the hardest.

In one of the largest anti-government protests in New Delhi in recent years, at least 50,000 people representing trade unions from the country's political parties, marched through the centre of the capital towards the parliament building.

In a sea of red flags and hats bearing their union name, protesters chanted slogans and carried banners calling on the government to provide food security.

Prices will now kill the common man, read one banner.

We get paid 100-125 Rupees ($2-3) a day. How are we going to survive on this if prices are so high? said Kailash Sain, who had travelled to the capital from the western state of Rajasthan.

We have come here so that our voices reverberate inside the house (parliament) and they can see what pain the common man is going through, said another demonstrator, Akhil Samamtray from Orissa.

Protesters arrived by bus and train from all over the country and the numbers were expected to rise.

PJ Raju, secretary of the Congress' trade union, told Reuters around 100,000 people from his party alone would be joining the protest.

The presence of trade union members from Prime Minister Manmohan Singh's ruling Congress party -- a rare instance of a protest against their own government -- is a telling sign of the concerns within the party about government policies that have been unable to tame inflation.

The government has looked increasingly helpless as it tries to introduce policies to rein in food prices, which analysts say have come far too late.

The Reserve Bank of India has raised interest rates seven times in a year to try and tame rising prices but has warned fiscal policies would be largely ineffective against rising food prices which stem largely from bad weather and problems on the supply side.

The protests also come only a day after Singh relented to months of opposition demands for a parliamentary probe into a multi-billion dollar scandal over sales of telecoms licences for kickbacks.

The scandals have piled enormous pressure on the reformist 78-year-old prime minister, seen as a lame duck who plays second fiddle to Congress party head Sonia Gandhi. Some believe further revelations could force him from power early and lead to an interim leader before a 2014 general election