Protests have broken out in Vietnam against plans for three new economic zones, amid fears that Chinese investors will dominate the areas.
Police reportedly detained more than a dozen people in the capital Hanoi and halted demonstrations in other cities.
Some carried anti-China banners, including one reading: "No leasing land to China even for one day."
The government proposed a law last year that would give foreign investors a 99-year lease on Vietnamese land.
The bill offers them greater incentives and fewer restrictions, in an attempt to promote growth in target areas.
The protesters suspect that the communist government will award Chinese investors leases in the three economic zones in the north-east, south-east and south-west of the country, and that this would be a pretext for Chinese control over the island of Van Don near their shared border.
Vietnam won a naval battle against a Mongolian fleet off the island in 1287, and some Vietnamese people fear their government will give it away amid tensions between the two countries over disputed territory in the South China Sea.
Prime Minister Nguyen Xuan Phuc last week told local media the 99-year term would be reduced, although he did not specify the new length.
And on Saturday, the government announced a vote on the draft legislation would be delayed to allow further scrutiny.
Demonstrators are also objecting to a cyber security bill, scheduled for a vote on 12 June. Human Rights Watch say it would give the government broad powers quash dissent online.
Roughly $5 trillion worth of global trade passes through the South China Sea annually, and a number of countries claim disputed islands in the area.
Vietnamese protests broke out in 2012 over the maritime dispute, although police quickly halted the demonstrations.