Four Britons are believed to have been arrested in Russia following protests outside the St Petersburg hotel where the G8 leaders are staying.
Last year's G8 summit in Gleneagles also saw protests
The anti-globalisation protesters from Network Against G8 (NAG8) are thought to be amongst a group of 10 arrested on Saturday and Sunday.
Members of NAG8 claim those arrested have been denied legal representation.
The Foreign Office said consular staff would investigate whether there had been any mistreatment.
The NAG8 group claims on its website that the arrested Britons are Welsh activists and 35 protesters were arrested in total.
It also claims these four have been denied access to lawyers and embassy officials and have also not been allowed water.
A spokesman for the Foreign Office said: "Consular staff are providing assistance and have been in touch with the police."
The G8, which meets annually, is made up of the world's richest nations - the US, Japan, Germany, France, Britain, Italy, Russia and Canada.