The annual awareness day drew particular attention this year with campaigners defending women's labour rights as Spain struggles with a 26-percent unemployment rate despite exiting recession in 2013.
It also reflected anger at a plan by the conservative government to end a right for women to abort on demand up to 14 weeks of pregnancy -- a reform that has yet to reach parliament.
Several thousand demonstrators marched in central Madrid and Barcelona with flags and drums, AFP reporters saw. Similar demonstrations were called in several other cities.
"No legislation in our wombs, no mistreatment of our bodies, no cutting of our rights," read one banner in Madrid.
Protesters denounced the abortion plan and the government's 2012 crisis labour reform, which they say is making it harder for working mothers to hold down a job.
In Spain, International Women's Day "this year is very special," said one protester, Angela Barrios Manjon, 66.
"The labour reform and the abortion law are against women," she added. "The right wing wants women to stay at home and look after the children."
"It is the first time I have come, but this year with the abortion reform it was more important than ever," said a protester in Barcelona, Lara Rubio, 23.
The government said it had approved a €3 billion ($4 billion) "strategic equal opportunities plan" to boost women's labour rights and protect them from violence.
"A woman can feel proud to know that her government is working for real equality for women," the ruling Popular Party's deputy leader Maria Dolores de Cospedal told a conference on Saturday.
"But a great deal remains to be achieved," for example in access to jobs and equal salaries, she said.
The deputy leader of the opposition Socialists, Elena Valenciano, retorted in a televised speech: "The Spanish right wing has never lifted a finger for women.
"Despite the trumpeting of the economic recovery, the job market for women in Spain is an ever-harsher reality."