The event, called SkirtBike, "aims to celebrate the independence and empowerment that cycling can provide to all women and to encourage those who do not already cycle to do so", organisers said.
"We want to show that it is possible for women to be elegant on a bike in order to promote cycling as a normal way of transport" to work, school or shopping, Oana Deliu, one of the organisers, told AFP.
In the capital Bucharest, hundreds of women defied pouring rain and cycled for several kilometres, wearing colourful skirts and dresses under transparent raincoats or holding an umbrella in one hand.
Unlike in Amsterdam or Copenhagen, where biking is an established tradition, urban cycling is a new trend in Romania.
"This event is very important because it shows people that more and more women are biking and it could be an example" for others, said 29-year-old Cristina Dumitru, who cycles every day to work.
"If more and more people would use bikes in Bucharest, the city would be cleaner," she added.
"Biking is a way to be different in a big city. You can customise your bike like you want and it is environment friendly," said Laura Balan, another participant.
Deliu has created an advocacy blog that promotes urban cycling for women and calls for better biking infrastructure in Bucharest.
Hundreds of women took part in the event in 11 other cities across the country, including western Timisoara, central Sibiu, northeastern Iasi and northwestern Cluj, police said.