President Salva Kiir is said to have ordered the army to occupy Freedom Square on Tuesday evening.
Mobilisers have been rallying the youth to turn up on May 15 to launch the protests against the regime over instability and other excesses.
Government spokesman says the youths are being funded by an unnamed foreign power to force a regime change.
South Sudan military troops occupied the Freedom Square late Tuesday evening in an attempt to prevent anti-government protests called by the youth.
The country's youth in diaspora and at home are mobilizing across the country and through social media demanding the toppling of President Salva Kiir's regime.
Many of the social media messages are emanating from a group calling itself South Sudanese for Change.
Among the grievances is the government's failure to restore stability in the war-torn country, abuse of human rights, corruption and intolerance of criticism.
The youth have called for a mass rally on May 15 in Juba to launch the nationwide protests with the ambition to oust Kiir and form a provisional government of national unity.
That date also happens to be emotive to the military as it commemorates its commissioning. Protests are also planned in Washington DC on May 20, to rouse up international interest.
The government has taken the threat seriously with sources saying President Salva Kiir, who is mediating on the Sudan crisis, ordered the occupation of the Freedom Square by the army on Tuesday evening.
The venue is close to the President’s Office and the National Assembly.
The square was condoned off to an extent that Africa Review reporters could not get close enough for a shot.
“It is likely that the army will extend their blockades across Juba to maintain order in the city," one source said.
One of the youth activists, Keluel Agok, however, was defiant.
He said the army would be overwhelmed as protests would be countrywide and in the diaspora.
“The leaders behind deployment of troops in Freedom Square will not control such an event. Juba square is not the only ground for the protest. Even in the diaspora mobilization is going on,” Mr. Agok said.
He said the army invasion of Freedom Square to prevent expected protests was a violation of the country’s constitution.
“That is a violation of the constitution by the government already,” he said.
But can the youth achieve what their counterparts in neighbouring Sudan and Algeria managed in seeing off dictators in a second round of Arab Spring which is gradually gaining appeal in other African countries.
Presidential hopeful and former Secretary General on External Relations of the ruling Sudan People’s Liberation Movement (SPLM) Suzanne Jambo believes so.
“Peacefully! South Sudanese youth and women have united and together we shall ascertain ourselves soon,” Ms Jambo said in a post on her official Facebook page.
she said.
The government is equally confident with its spokesman Makuei Lueth warning it would "deal with anybody who protests.”
He pointed a finger at unnamed foreign instigators who he claimed were funding regime change in Juba.
“What happened in Sudan should not be copied in South Sudan,” he said.