SAO PAULO, April 4 (Reuters) - Indigenous protests and poor roads have disrupted shipping of Brazil's bumper soybean crop this week via the river port of Miritituba in the Amazon rainforest, where global grains traders including Cargill and Bunge have important operations.

Demand for soy from Brazil, the world's largest producer and exporter, has surged in No. 1 consumer China as traders brace for a trade war discouraging Chinese imports of U.S. soybeans.

Miritituba loaded some 15 million tons of soy and corn last year onto barges bound for larger shipping ports down river, representing more than a tenth of Brazil's total exports of those grains. Volumes at the port are expected to rise around 20% this year.

Protesters from the Munduruku people have been blocking a key stretch of the Transamazonian Highway near Miritituba at certain hours of the day to pressure Brazil's Supreme Court to overturn a 2023 law aimed at limiting their land rights.

That has worsened backups along an unpaved five-kilometer stretch of the road. Trucking group ANATC said the traffic has left some cargos waiting three days to unload at Miritituba.

AMPORT, which represents the largest firms shipping from the terminal, said truckers with pre-scheduled access have not suffered those wait times at the port.

Still, AMPORT President Flavio Acatauassu estimated each hour of the protesters' blockade prevents at least 12,000 tons of soybeans from arriving at the terminal.

Via Brasil BR-163, which administers 1,009 kilometers (627 miles) of the highway linking farms in Mato Grosso state to the river port, said a new access will be built when courts give it permission to expropriate certain areas.

Frustrations have boiled over into violent episodes between truckers and Indigenous protesters, according to a statement from Munduruku representatives.

"Our fight is peaceful, but we have been suffering attacks and threats from truck drivers, including insults, stone throwing, gun shots and dangerous driving," they wrote.

Rafael Modesto, a lawyer for the Indigenous Missionary Council, which argues for Indigenous interests before the Supreme Court, said the protest reflects fears among native peoples about losing their lands to an advancing farm frontier.

Brazil's powerful congressional farm lobby has been at odds with the Supreme Court over a proposed cut-off date for new reservations on lands where Indigenous people were not living in 1988.

"We believe that, if any proposal that changes the text of the Constitution goes through, demonstrations like this one may become more frequent all over Brazil," he said. (Reporting by Ana Mano Additional reporting by Manuela Andreoni Editing by Brad Haynes and Alistair Bell)