"As long as the protests remain peaceful we will not use force but if protesters try to breach the prohibited zone we will take all necessary actions."
The Maoists quit the government in May after the president stopped them from sacking the head of the army.
Led by the former prime minister Prachanda, the Maoists have said that the president had undermined the supremacy of the civilian government in stopping them from acting.
Series of protests
Thursday's demonstration was part of a fortnight-long series of nationwide protests being held by the Maoists, who fought a 10-year civil war against the state that ended in 2006.
|The Maoists quit the government after a call to sack the army chief were refused[AFP]
The protest had started in celebratory mood, with activists dancing and playing traditional music as they handed round dishes of rice and curry.
But the atmosphere turned angry as the clashes broke out and one television station showed police using axes to destroy musical instruments left behind when the demonstrators fled.
Krishna Bahadur Mahara, a senior Maoist leader, who was among those wounded in the clashes on Thursday, said the blockade would mark the end of the the group's two-week long anti-government protest.
"We are now giving the government and other parties an opportunity to look into our demands," he told AFP news agency.
"The ball is in the government's court."
The Maoists emerged as the largest political party in elections last year after agreeing to a peace deal in 2006 and got the 239-year-old monarchy abolished through a special assembly which turned Nepal into a republic.
But they quit the government after the dispute with the president.