By Ruma Paul

DHAKA, Jan 9 (Reuters) - Bangladesh has stepped up a crackdown on the main opposition party, arresting its vice-president amid anti-government protests that have left seven dead and injured hundreds.

A standoff between Prime Minister Sheikh Hasina and Begum Khaleda Zia, leader of the opposition Bangladesh Nationalist Party, has worsened steadily since Monday, when protests erupted over last year's general election, which the BNP boycotted.

Police arrested Shamsher Mobin Chowdhury of the BNP at his home in Dhaka, the capital, on Thursday night.

"A Dhaka court granted five days of remand to Chowdhury in a case of attempt to murder a ruling party lawmaker," Joint Police Commissioner Monirul Islam told Reuters on Friday.

The renewed tension between the political rivals raises the spectre of a long, destabilizing spell of unrest for Bangladesh and its economy, though some question whether Khaleda and her party can bring down the government.

Khaleda has demanded that Hasina give up her post, with a new vote to be held under a neutral administration.

The prime minister has rejected these demands. Instead, her government has tightened its grip. Human rights groups have expressed concern about the arrest of hundreds of opposition supporters, excessive police force and a media clampdown.

"The government's indiscriminate use of force, arbitrary arrests, and censorship will only inflame an already tense situation," Brad Adams, Asia director at Human Rights Watch, said in a statement.

On Thursday, police filed a sedition case against Tareque Rahman, Khaleda's exiled son who lives in Britain, for statements about Bangladesh's first prime minister, and Hasina's father, that police called "abusive".

They also filed a sedition case against Abdus Salam, the owner of a private TV channel, who was arrested on charges of pornography, for airing a "false, fabricated and instigating speech" by Rahman.

The high court has banned media coverage of Rahman's speech, prompting protesters to attack the homes of the judges who gave the order.

The BNP says Khaleda has been confined to her office in Dhaka's diplomatic enclave since the weekend, with thousands of its supporters arrested. Police could not confirm the figure.

The government denied Khaleda was being held against her will, saying the security was for her protection.

"We are doing it to ensure her security," Asaduzzaman Khan, junior minister for home affairs, told reporters.

BNP Joint Secretary General Ruhul Kabir Rizvi condemned Thursday's arrest and urged supporters to take to the streets.

"The government will have to leave power in the face of this mass movement," Rizvi told reporters. "It is now just a matter of time." (Editing by Krista Mahr, Douglas Busvine and Clarence Fernandez)