People are queuing up outside banks across India to exchange 500 and 1,000 rupee notes after they were withdrawn as part of anti-corruption measures.
Indians will be able to exchange their old notes, which stopped being legal tender at midnight on Tuesday, for new ones at banks until 30 December.
The surprise move is part of a government crackdown on corruption and illegal cash holdings.
Banks were shut on Wednesday to allow them enough time to stock new notes.
There are also limits on cash withdrawals from ATMs.
The BBC's Yogita Limaye in Mumbai says there have been chaotic scenes outside many banks.
"At one branch there are at least 100 people waiting outside. Worries that banks will run out of currency notes are driving people to queue up outside them. And the fears may not be unfounded. At least two banks I've been to since the morning are only allowing deposits and not exchange because they have run out of new currency notes," our correspondent says.
There was a similar picture in Delhi. The BBC's Vikas Pandey says: "Almost every bank I have visited in old Delhi has long lines. People, mostly businessmen, are worried. There seems to be a lot of confusion over what people can and cannot do with their cash."
The banned currency notes represent 85% of the cash in circulation in India, which is an overwhelmingly cash-based economy.
Media captionOld 500 and 1,000 rupee notes are no longer in circulation
People stand in line as they wait to enter a bank in Kolkata, India, November 10, 2016
A line outside a bank in Kolkata
An Indian man displays a new 2000 rupee note after exchanging his old 500 and 1000 rupee notes at a bank in New Delhi on November 10, 2016
New 2,000 rupee denomination notes were introduced on Thursday
Indian people queue outside the Reserve Bank of India to deposit and exchange 500 and 1000 currency notes, in New Delhi on November 10, 2016.
People queue outside the Reserve Bank of India in the capital to deposit and exchange banknotes
Meanwhile, Indian social media has been talking of little else.
The top trend on Twitter India has been #CashCleanUp with tweets ranging from the frustrated to the humorous, as many Indians came to terms with the fact that much of their day would be spent in queues.
Before you go to bank
New 2,000 (about $30; £24) and 500 rupee denomination notes to replace those removed from circulation will be injected into the economy over the next "three to four weeks", Finance Minister Arun Jaitley has said.
The move is designed to lock out money that is unaccounted for - known as "black money" - which may have been acquired corruptly, or is being withheld from the tax authorities.
Finance Secretary Shaktikant Das warned people with large amounts of hidden cash that banks would closely monitor the exchange of old notes for new ones.
The government says the move will flush out tax evaders and that all old notes deposited in banks will be subjected to tax laws.
How long have people got to change their old notes?
The 500 and 1,000 rupee notes are the highest denomination notes in the country and are extremely common in India. Airports, railway stations and hospitals will only accept them until 11 November. People will be able to exchange their money at banks between 10 November and 30 December.
How much 'black money' is there in circulation?
The actual figure is unclear but correspondents say the issue of "black money" is a huge problem in India. The idea here is to lock out money that is unaccounted for and make it visible for tax purposes - banks will be happy to exchange a few thousand rupees, but will be asking questions of those who turn up with hundreds of thousands or millions in currency.
Is there a limit on the amount an individual or household can cash in?
It seems not. An individual can put as much as he or she likes into the bank - but withdrawals are limited so the banking system may end up being flooded with cash.
Indian government flyer featuring Prime Minister Narendra Modi on its rupee move
The government has issued flyers explaining the changes
Government guidelines say it is possible to exchange 4,000 rupees - but it is not clear if this is per day or in total. If there is a legitimate explanation for the cash, the authorities say, it will be possible to exchange it.