Demonstrators clashed with police in Vietnam in protests against plans for new economic zones that some fear will be dominated by Chinese investors.
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.
China once colonised Vietnam, the two countries fought a border war less than 40 years ago, and Vietnam contests Chinese control of a number of islands in the South China Sea.
As a result, says BBC South East Asia correspondent Jonathan Head, Vietnam’s leaders must always tread a delicate line between maintaining relations with their powerful neighbour, and avoiding provoking anti-Chinese sentiment in a fiercely nationalist population.
Protesters bring China issue to the fore
By Giang Nguyen, editor of BBC Vietnamese
This is the biggest challenge for Vietnam’s Prime Minister Nguyen Xuan Phuc since his government was sworn in more than two years ago with a pledge to stamp out corruption and revitalise the economy.
The three special economic zones are meant to be “mini Singapores” – business-friendly environments complete with high-tech hubs.
But Mr Phuc appears to have underestimated deep-seated resentments against China, and the speed at which protesters can utilise social media to organise street marches in cities including Hanoi and Saigon.
While some are fearful of a perceived Chinese influence in Vietnam under the economic zone proposals, others are concerned about plans for a new cyber security bill. The latter has angered Facebook users in particular, who fear the authorities will be given too much power, while online surveillance could become the norm.
Next week, two parliamentary sessions – on 12 and 15 June – are scheduled for voting on the two draft bills and protesters have said they will take to the streets again until – or unless – the government abandons them both.
Vietnam won a naval battle against a Mongolian fleet off Van Don island in 1288, 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 says it would give the government broad powers to 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.
Vietnam has seen protests over the maritime disputes in recent years, including in 2014, when Chinese citizens fled the country in their thousands after violence targeting foreign-owned businesses.