Security is tight during two-day meeting that was expected to be dominated by discussions about how the G-20 nations will respond to the deadly Paris attacks, claimed by the Islamic State group.
Demonstrators were being kept miles away from the venue at a secluded seaside resort some 40 kilometers (25 miles) from Antalya city.
A group of some 500 youths belonging to a Turkish nationalist association gathered in the city, holding up card-board effigies of US President Barack Obama and denouncing US interventions in the Middle East.
Hundreds of members of Turkish left-wing groups and trade unions later held another protest denouncing the organisation which gathers the world's wealthiest economies.
Also Read
They marched in central Antalya carrying a banner that read in Turkish and in English: "Killer, colonialist, imperialist war organisation G-20 get out!"
Police detained dozens of demonstrators after one of the demonstrators threw fireworks at police while the crowd was dispersing.
Earlier, police detained four protesters who wanted to walk to the venue of the G-20 summit to deliver a letter to participants.
Separately, members of Turkey's ethnic Uighur community also gathered in the city to protest China's treatment of the Muslim minority.
Chinese President Xi Jinping is among the summit participants.