Two activists from the main opposition Bangladesh Nationalist Party have been shot dead in clashes with ruling party supporters after their leader called for protests on the first anniversary of elections her party boycotted, police said.

Monday's clashes came as BNP's leader Khaleda Zia remains confined to her office in the capital Dhaka in what is seen as attempts by the authorities to prevent her from staging protests.

The two activists were killed in the northern town of Natore on Monday morning in what police said were clashes with Awami League supporters.

The victims were identified as men in their 20s and shot by assailants on motorbikes. 

The attack happened as authorities stepped up their siege of Zia's upmarket Gulshan office, parking trucks laden with sand and bricks to block the road leading to the office.

Al Jazeera's Tanvir Chowdhury, reporting from Dhaka, said Zia was trying to force her way out.

"Khaleda Zia is planning to come out of her office any time now, [but] the main gate is locked and [the] entire area is cordoned off and barricaded by trucks," he said.

Police said the office had been cordoned off "In an effort to step up her security".

Opposition threat

Our correspondent said Rawshan Ershad, the leader of parliamentary opposition, was threatening to withdraw her ministers from the cabinet.

Activists set fires close to the home of Zia [AFP]

Ershad made the comments while addressing a news conference in Dhaka at the parliament building on Monday. It was not clear whether the threat was linked to restrictions placed on Zia.

Local media reported clashes in various parts of the country that have left dozens injured by rubber bullets fired by security forces.

There were also media reports of crowds setting fire to the office of the ruling Awami League in the district of Rajshahi, but the reports could not be indepently verified.

Zia has urged activists to take to the streets in their thousands as part of a campaign to force Prime Minister Sheikh Hasina to hold fresh multi-party polls, describing Monday as "Democracy Killing Day".

"She has urged people to join a mass rally today. She would also try to join the protest," Zia's spokesman Maruf Kamal Khan told reporters.

Disputed election

Hasina, who has been in power since 2009, was re-elected on January 5, 2014, in what was effectively a one-horse race after the BNP and around 20 other opposition parties boycotted the polls over rigging fears.

Zia's boycott was sparked by her arch rival's refusal to step down ahead of the election and allow the contest to be organised by a neutral caretaker administration which has organised previous polls.

The boycott by the BNP and its allies meant a majority of members in the 300-seat parliament were returned unopposed, ensuring Hasina's Awami League party another five years in power.

The opposition has since maintained that the poll was a sham.

"It has several times tried to start up some agitation since but each time the government has suppressed it by rounding up and jailing their activists," Al Jazeera's Maher Sattar reported from Dhaka.

"This time has not been too different. Clashes are still sporadic though, nothing like 2013 yet."

In the January 2014 election, voting was overshadowed by firebomb attacks on polling booths and clashes between police and opposition activists on and before election day. About 500 people were reportedly killed in the run-up to the poll.

Many of the BNP's top leaders have since either been detained or charged in connection with the election violence, hampering efforts to press their case for new polls.

Source: Al Jazeera and agencies