Speakers and organizers were arrested (though ultimately charges of high treason were dropped), but also arrested were journalists who reported on the event and published their accounts in newspapers.
As appalling as it might seem to the Western World now, freedom to protest, freedom of speech and of the press, were outlawed in 1819. Earlier episodes of social unrest convinced Lord Sidmouth, the Home Secretary, that revolution was in the wind. (Remember, the French Revolution would have been fresh in the minds of the British Peerage). Parliament passed the Gag Acts suspending Habeas Corpus, banning seditious meetings and the printing of seditious documents. This meant anyone could be arrested and held without charges, and it meant the limiting of free speech and a free press. Quite extreme measures. After Peterloo, as the episode became known, Parliament passed the even more repressive Six Acts.
Even so, many historians consider Peterloo a turning point in the fight for freedom and political reform in Great Britain. The horror of the government attacking peaceful citizens changed public opinion and ultimately led to the right of the citizenry (alas, men only for several years) to vote.
What books have you read which involve the Peterloo Massacre? It would make great drama!