I was there yesterday, arriving a couple of hours in to the event, and there were far more than just 60 people protesting! There were hundreds of people at Speaker's Corner itself (even two hours in) - most just sitting down and listening to music or having a picnic. Some had banners, specially made t-shirts and signs. Several had megaphones and there were people with drums. We didn't see Piers Corbyn, probably being too late to witness his arrest.
A spontaneous march started and, again, I would say that one or two hundred people were involved. It was after the march started that the police closed in, in very large numbers marching in formation across the grass, and then started ramping up the arrests.
It wasn't specifically an "anti-vaxxer protest" - there were a wide variety of people of all ages and all with their own take on why they feel lockdown is wrong.
In other parts of Hyde Park, there were also vast numbers of people sitting out in large groups and enjoying the weather. (Anti)social distancing was completely ignored. I didn't witness any arrests, or even people being spoken to, in other areas although the police were driving around in their vans and passing on horses to make their presence known. What took place at Speaker's Corner was peaceful so there really was little difference compared to the actions of people in other areas of the park who didn't get troubled by police - the only difference being the latter were ignoring lockdown in a quiet rather than more obvious and vocal way.