The Metropolitan Police said the criminal damage took place during the protest in Parliament Square, Westminster, on Saturday.
Thousands of people had gathered for an “emergency demonstration” following the Supreme Court ruling earlier this week on the definition of a woman.
The words “Trans rights are human rights” were written on a statue of South African statesman Jan Christian Smuts.
The Met Police said officers were trawling CCTV footage and no arrests have been made as yet.
Chief Superintendent Stuart Bell said: “Criminal damage and vandalism like this has no place on the streets of London and spoils the area for locals and those visiting.
“While the police support the public’s right to protest, criminality like this is senseless and unacceptable. We are pursuing this and will take action against those responsible.”
Mr Bell said there were plans to remove the graffiti “but this requires specialist equipment and we are confident this will be done shortly”.
“We are keen to speak to those who saw anything on the day and urge anyone with information, pictures or footage to come forward,” he added.
The force has not shared details of which seven statues were defaced. Parliament Square is home to 12 statues of political figures including Winston Churchill, Abraham Lincoln and Mahatma Gandhi.
The Met said it was also dealing with a number of complaints from the public about signs and images shared on social media that were reportedly displayed at the protest.
The force said while officers were investigating the displayed signs and images, they had so far proved to be from historic events, did not take place in London, or did not constitute a criminal offence.
The protest followed a long-awaited judgment delivered on Wednesday, when the UK’s highest court ruled the terms “woman” and “sex” in the Equality Act 2010 “refer to a biological woman and biological sex”.
It essentially means trans women who hold gender recognition certificates are not women in the eyes of the law.
This means transgender women with one of the certificates can be excluded from single-sex spaces if “proportionate”.
The UK government said the unanimous decision by five judges brought “clarity and confidence” for women and service providers.
