Finnair, the national carrier, has cancelled 16 domestic flights and delays are expected.
Trains and city buses have stopped running, but ferries, including those to Sweden and Estonia, are operational.
Strikers are protesting against government cutbacks, including limits to benefits and overtime pay.
Talks on a collective agreement on wages and working hours collapsed, leading to the strike, Associated Press reported.
Last week, Finnish Prime Minister Juha Sipila announced plans aimed at reviving the eurozone member's economy after three years of recession.
The plans included cutting back holidays, reducing pensioners' housing allowances, and reductions in  employees' overtime and Sunday pay.
"The Finnish state has contracted debt at a rate of almost a million euros (£730,000) per hour for seven years, day and night, every day of the week.  We cannot continue like this," Mr Sipila said.
Police and organisers expect thousands of people to demonstrate in Helsinki, the AFP news agency reported.
A widespread anti-austerity strike in Finland has shut down ports and disrupted flights in the country, news agencies have reported.