NEW DELHI, Dec 13 (Reuters) - India reported its lowest tally of active COVID-19 cases in 18 months on Monday, but a sharp drop in the use of protective face masks is causing concern after a rise in the number of infections with the Omicron variant. Many people have been standing or sitting close to each other without masks, or covering only their chins, at big rallies held by political parties in several states before elections. Something similar happened before the Delta variant ravaged India from April. Cases have come down sharply since then, with an active COVID-19 total of 91,456 as of early Monday, the lowest in 561 days, according to the Health Ministry. But cases of the Omicron variant have risen to at least 36 in India, and accounted for 3% of the virus sequences analysed in the country in the past two weeks, with Delta accounting for the rest. Health authorities have been urging people to cover their mouths in public. "The falling graph of mask use could cost us," top Indian health official Vinod Kumar Paul told a recent news briefing. "Mask is a universal vaccine, works on every variant." Mask-wearing in public has fallen to levels last seen in March, before the second wave of cases, data from the Institute of Health Metrics and Evaluation shows. Current mask-wearing is estimated at 59%, nearly the same as in March, having peaked at 81% in May. The country reported 7,350 new COVID-19 cases on Monday, taking the total to 34.69 million. Deaths rose by 202 to 475,636.