The legislation, which is moving through Parliament, would let police break up peaceful demonstrations if they think they're disruptive.
Since the proposed law was brought before parliament last month, there have been sporadic demonstrations across the country, with Saturday's rallies being part of what organisers said was a national weekend of action.
Several women addressed the crowd and shared personal experiences of suffering abuse and being drugged.
"At this stage, 26 people have been arrested for a variety of offences including assault on police and breach of the peace", the Metropolitan Police said in a statement, adding that the policing operation is still ongoing and that the number of arrests is likely to rise.
The protests have been sparked by the proposed Police, Crime, Sentencing and Courts Bill, would give police in England and Wales more power to impose conditions on non-violent protests, including those deemed too noisy or a nuisance, with those convicted liable to fines or jail terms.
Placards carried by protesters included slogans such as "educate your sons", "misogyny is the virus", and "girls just wanna have fun-damental human rights".
Commander Ade Adelekan, who described it as a "challenging day for officers", said: "The vast majority of people who turned out in central London today did so while adhering to social distancing".
Over the past several weeks, as COVID-19 restrictions on mass gatherings have become laxer, "Kill the Bill" protests have taken place across the United Kingdom against a government-proposed bill that would give more powers to police at non-violent protests.
"As protest is criminalised and our fake democracy is pushed further towards authoritarianism, we seek to find where we can come together with other movements to tackle the common causes that affect us all", said Alanna Byrne from environmental group Extinction Rebellion.
Hundreds of people also marched through Newcastle city centre, and other places that saw protests of varying scales included Birmingham, Liverpool, Newcastle, Bournemouth, Brighton, Weymouth and Luton.
He said the proposed introduction of the news law was a "very unsafe, slippery slope" undermining the right to protest.
They also referred to the Home Secretary, chanting "F*** Priti Patel".
Many took the knee at the Civic Centre and held a minute's silence for victims of oppression, after which a round of applause broke out.