The tractors, many travelling at less than 40km an hour, began entering Oslo in the morning, during the city's rush hour.
From 12.45, the farmers plan to mount a procession through central Oslo to the country's parliament in protest at the size of the government's proposed increase to the subsidies they enjoy.
Nils Bjørke, the head of the farmers' union, stressed that his members aimed only make their presence felt, not to shut down the city .
"There is no point in us crippling the entire of Oslo, so there aren't as many tractors as many of us wanted to have," he said. "But it's going to be noticed by the cars on the road."
The union of smallholders threatened to use the farmers time-honoured political weapon, and deposit manure in front of the parliament building.
"You will see if there is any manure dumped outside the parliament, the Norwegian farmers and smallholders union head Merete Furuberg told NRK on Monday.
The farmers are protesting against the government's decision to limit the increase to subsidies to 150 million kroner, a 10th of what farmers demanded.