Shouting anti-government slogans as they sat astride their tractors, thousands of farmers clogged up a major highway into India's capital Thursday as they warned their month-long protest would not stop until recent agriculture reforms are repealed.

Tens of thousands of farmers have held sit-ins on key roads into New Delhi after being blocked from marching into the capital in late November over laws deregulating their industry.

"Down with (Prime Minister Narendra) Modi! Down with the government!," they chanted as a colourful, kilometres-long convoy of farming machines snaked along the expressway.

"What you think, will these many tractors open Modi's eyes? Do you think it will put pressure on Modi government?," a passing farmer said to an AFP photographer at the highway.

There was minimal police presence on the road and although officers tried to restrict the tractors to two lanes, they were unsuccessful. 

The farming unions leading the protests said the rally was a dry-run for a much bigger tractor parade to cripple the capital on January 26 when India holds its annual Republic Day celebrations and Modi delivers a keynote speech in Delhi.

Modi has said the changes would allow farmers to sell to private buyers instead of just at state markets.

But the demonstrators -- mostly from states in northern India near Delhi -- fear that under the new system, large corporations would squeeze them for profits and destroy their livelihoods.

Several rounds of talks between the farmers have yielded no progress. The next round of talks will take place on Friday.