The BSE Sensex edged lower on Friday as caution ahead of the launch of a unified goods and services tax (GST) hit auto makers, with benchmark indexes heading for their first monthly loss this year.
The revised tax structure will kick in from midnight Friday, marking India's biggest tax reform since independence, unifying its $2 trillion economy and 1.3 billion people into a common market.
Weakness in Asian peers following losses on Wall Street overnight also hurt sentiment. The MSCI's broadest index of Asia-Pacific shares outside Japan was down 0.7 percent.
"There is some kind of profit-booking and sideways consolidation in the market, so most sectors are volatile," said Siddharth Sedani, head and vice president, equity advisory at Anand Rathi.
The broader NSE Nifty was down 0.12 percent at 9,492.35 as of 0702 GMT, on track to post its third week of losses. The index was down 1.4 percent for the month, snapping a five-month winning streak.
The benchmark Sensex was 0.14 percent lower at 30,815.72. The index was down 1.03 percent for the week and 1.04 percent for the month.
Auto makers were among the top losers on worries that GST would push up prices of cars and lead to a decline in sales. The Nifty Auto index lost as much as 0.9 percent, with Tata Motors Ltd falling as much as 1.6 percent.
IndiGo owner InterGlobe Aviation Ltd slid as much as 5.7 percent after falling 2.2 percent on Thursday after the company expressed interest in buying a stake in Air India after the government approved plans to privatise the debt-laden state-owned carrier.
Central Depository Services (India) Ltd surged as much as 80 percent on its trading debut, after raising 5.2 billion rupees ($80.43 million) in the country's most heavily over-subscribed initial public offering this year.