Some 30,000 workers at two of India's biggest state-run oil firms, HPCL and BPCL, launched a strike on Tuesday to oppose privatisation of the refiners, a senior union official said.

But analysts said they believed the three-day walkout would be unable to derail the sales, India's biggest privatisation initiative in its more than decade-old sell-off drive that has been dogged by political and union opposition.

And shares of the two companies were up in morning trade, buoyed by a Supreme Court ruling on Monday that the government could proceed with privatisation of the refiners despite a suit pending before it challenging the selloff, analysts said.

The two cash-rich companies, Hindustan Petroleum Corp Ltd and Bharat Petroleum Corp Ltd, hold a 40 per cent share of India's oil product market.

The National Secretary of the Centre of Indian Trade Unions, one of the nation's biggest unions, said the workers were demanding "the move to privatise these national assets be reversed" but the strike was not aimed at halting operations.

"The strike has taken off very well. Twenty-six unions have come together. Around 30,000 workers are participating in the strike," said Swadesh Debroye, CITU national secretary.

But analysts said it was unlikely that labour protests could block the sale of the companies that the government said in February it wanted to complete in six-to-eight months.

"These cannot stall the privatisation process," said Bahrain-based Hemant Kulkarni, a TAIB Bank portfolio manager.

Shares in BPCL were up 2.1 per cent at Rs 220.85, while Hindustan Petroleum Corp rose 1.4 per cent to Rs 298.60 on the Bombay Stock Exchange, outperforming the benchmark index which was down 0.46 per cent by late morning. "I think the Supreme Court ruling is a big trigger for the two stocks," Kulkarni said.

An HPCL spokesman in Mumbai said that the management staff reported to work after a court banned them from striking but non-management staff had not turned up at its headquarters. BPCL had said it has taken steps to minimise disruption to supplies.

Debroye said the strike would have an "adverse impact at depots but nothing will come to a standstill as the operations are all mechanised. It is not our purpose also -- our purpose is to give a serious warning to the government on privatisation."

The government plans to sell a controlling 34.01 per cent stake in HPCL to a strategic partner and shed majority control in BPCL with a public issue of 35.2 per cent of the company.

Oil giant Chevron Texaco is among the bidders for a stake in HPCL along with Malaysian state fuel giant Petronas.