Vietnam has arrested at least four people this week in the lead-up to its national day on Sunday, state media said, accusing them of attempting to sabotage national security.
The one-party communist state frequently jails and convicts critics, especially since a hardline administration took power in 2016.
Authorities have warned citizens not to heed calls to protest during Sunday's independence day celebrations, vowing harsh penalties for anyone who causes "social instability".
On Saturday, police in the southern province of Can Tho arrested two men over their calls on Facebook for demonstrations, according to a report by state-run media.
Doanh Khanh Vinh Quang, 42, and Bui Manh Dong, 40, had also published posts "defaming party, state and senior leaders", the report said.
The most high-profile detention was of an alleged member of the US-based anti-government group Viet Tan, which Vietnam considers a terrorist organisation.
Le Quoc Binh, 44, was nabbed Wednesday in central Binh Dinh province in possession of seven rifles and 500 bullets he had brought in from neighbouring Cambodia, according to the Ministry of Public Security.
The cache of weapons was intended to "cause unrest on national day", it said, adding that Binh possessed many "anti-state documents".
A 38-year-old shrimp farmer, Nguyen Ngoc Anh, was also arrested on Thursday in southern Ben Tre province for planning a national day protest.
It remains unclear if the four men are connected.
At least 28 dissidents and bloggers have been convicted in Vietnam since the beginning of the year, according to a list tallied by AFP.
Among them, outspoken activist Le Dinh Luong was sentenced to 20 years -- the harshest penalty levied in recent years -- for his alleged role in calling for an election boycott and inciting environmental protests.
National day marks the declaration of independence on September 2, 1945 by Ho Chi Minh, the founder of the Vietnamese Communist Party.