Thousands of supporters of two opposition parties demanding the resignation of Pakistani Prime Minister Nawaz Sharif besieged the country's parliament on Wednesday while it was in session.

The protesters, who entered a high security area known as the "Red Zone" in Islamabad late on Tuesday night, blocked the two main gates of the parliament to stop members of the National Assembly, the lower house, from leaving after the session.

Sharif is also attending the meeting in the parliament house to discuss the ongoing protests in the country.

Pakistan Tehrik-i-Insaf Chairman Imran Khan and Pakistan Awami Tehrik chief Tahir ul Qadri led two separate marches from the eastern city of Lahore on Aug 14 to Islamabad to press the government to fulfill their demands, including the resignation of Sharif.

Khan, whose party has 34 seats in the lower house, alleged that Sharif's party managed to win the 2013 general election through mass vote-rigging.

Khan has been demanding a free and fair probe into the alleged rigging by a panel of the country's top court officials after Sharif resigns.

Meanwhile, PAT workers have been protesting in support of their 10-point agenda, which calls for the resignation of Sharif and Chief Minister of Punjab province Shabaz Sharif and reform of the system.

The PAT also demanded the registration of a murder case against Sharif and his brother, CM Punjab, over the police shootout with protesters on June 17 in Lahore, during which 14 PAT members were killed and 84 injured.

Earlier on June 16, a session court in Lahore ordered registration of a murder case against Sharif and the Punjab along with 19 others, including ministers and police officials, for the killing of 14 protesters.

No case has yet been registered against anyone after the court orders, enraging the protesters.

On Tuesday evening, Pakistan's federal government handed over security of important government buildings to the Pakistan army, under Article 245 of the Constitution, hours before the protesters started their march to the Red Zone.

The protesters removed all blockades from the routes and ignored all warnings by the police and entered the Red Zone.

At least one police officer and four PTI workers were injured during the march.

Besides 700 army troops, thousands of paramilitary troops and police officers are also present in the area to deal with any situation.

Members of the parliament condemned the protesters' siege, saying it is against the political and democratic norms to go to such extremes over political demands.

China on Wednesday urged both sides in Pakistan to resolve disputes in a peaceful way as the country's political crisis grows.

"As a good neighbor, China sincerely hopes that all parties concerned in Pakistan will proceed from the fundamental interests of the country and the people to resolve the crisis in a peaceful way so as to jointly safeguard the stability of the country," Foreign Ministry spokesman Qin Gang said in a statement.

Pakistan's army called on Wednesday for dialogue to resolve the current situation after the protesters entered the Red Zone.

Major political parties have called for patience and dialogue to resolve the political deadlock.

But all efforts at dialogue have failed so far.