Pakistan's Imran Khan leads big protest rally in Islamabad

Thousands of protesters have gathered in Pakistan's capital as part of a big rally from Lahore to Islamabad, led by opposition politician Imran Khan, the BBC reported. 

Mr Khan demanded that Prime Minister Nawaz Sharif step down, alleging rigging in the 2013 elections.

A separate convoy led by anti-government cleric Tahirul Qadri, who is calling for a "green revolution," also arrived in Islambad.

Last year's election saw the first civilian government transfer of power.

There was tight security in Islamabad as the protesters arrived in poor weather by truck, jeeps and cars after travelling the 350km (218 miles) from Lahore to Islamabad.

Mr Khan, who heads the opposition Pakistan Tehreek-e-Insaaf party, said that the government was illegitimate, citing fraud and a failure to organise a probe into the elections.

He called for new elections, vowing to continue a sit-in until Mr Sharif resigns.

Police estimated the crowd was 60,000 strong.