Venezuela President Nicolas Maduro has threatened to close the border with Colombia as opposition leader Juan Guaido and some 80 lawmakers ran a gauntlet of roadblocks trying to receive humanitarian aid.
Guaido, who is recognised by dozens of countries as Venezuela's legitimate head of state, was poised for a showdown with Maduro's government by attempting to bring in food and medicine from neighbouring countries.
Maduro denies there is a humanitarian crisis and said on Thursday he was considering closing Venezuela's key border with Colombia, having already decided to close the country's other main border with Brazil from 8pm.
This step would effectively shut off any legal land access.
The government has said soldiers will be stationed at official crossing points to repel any "territorial violations", although the opposition could attempt to cross anywhere along Venezuela's porous borders.
"I charge (Colombian President) Ivan Duque with any violence that might occur on the border," Maduro said in televised comments.
Venezuela has already closed its maritime border with the Dutch Caribbean islands of Aruba, Curacao and Bonaire.
Opposition lawmakers set off from Caracas in a convoy of buses just after 10am on a 800km road trip to the border with Colombia. Crowds formed alongside a main highway out of the capital, waving Venezuelan flags.
A roadblock at a tunnel some 100km along the main road forced several buses to stop. Lawmakers trying to get through scuffled with soldiers in riot gear at the tunnel's exit, TV footage showed.
"We have a commitment and that is to reach the border. We will try to get as far as we can," lawmaker Mariela Magallanes told Reuters.
Magallanes said her vehicle managed to pass through the tunnel after being stuck for several hours but other buses remained behind.
Lawmakers said Guaido's vehicle continued but his exact location was being kept a secret due to security concerns.
Guaido says 300,000 Venezuelans are at risk of death due to food and medicine shortages, while US Secretary of State Mike Pompeo vowed Washington would not back down from the aid plan.
The oil-rich Venezuelan economy has halved in size in five years.
Maduro accuses the Trump administration, which has levied crippling sanctions against his government, of seeking to force his ouster.
Meanwhile, the office of US Vice President Mike Pence said he would fly to Colombian capital Bogota on Monday to discuss the crisis with leaders of the regional Lima Group of nations.
Guaido invoked the constitution to assume an interim presidency on January 23 and denounces Maduro as an usurper. Maduro still retains the support of powerful nations like Russia and China, as well as the backing of the military.
Guaido has offered amnesty to military officers who disavow Maduro, though few have so far done so.
