Baghdad, Iraq ? Iraq’s prime minister set in motion the biggest security crackdown in Baghdad since the U.S.-led invasion, with 75,000 Iraqi and U.S. troops to deploy across the strife-prone capital starting today.

Prime Minister Nouri al-Maliki also announced plans for an extended curfew and a weapons ban, saying he would show “no mercy” to terrorists six days after al-Qaida in Iraq leader Abu Musab al-Zarqawi was killed in a U.S. airstrike northeast of Baghdad. The government did not say how long the crackdown would last.

The announcement came as radical anti-U.S. cleric Muqtada al-Sadr planned a demonstration today in Baghdad to protest President Bush’s surprise visit to the capital on Tuesday.

As dawn broke today over the Iraqi capital, only a few new police checkpoints had been added to those already functioning. It was not immediately clear whether others had gone up in Baghdad or its outskirts.

Al-Zarqawi’s successor, identified by the nom de guerre Abu Hamza al-Muhajer, vowed to defeat “crusaders and Shiites” in Iraq and said “holy warriors” in the country were stronger than ever, according to a Web statement posted Tuesday – the first from the new leader.

Underlining the threat, explosions struck oil-rich Kirkuk, killing at least 16 people. Kirkuk police Brig. Gen. Sarhad Qader said the attacks in the city 180 miles north of Baghdad were believed to be “a reaction to avenge the killing of al-Zarqawi.”

Security officials said 75,000 Iraqi and multinational forces would be deployed throughout Baghdad today, securing roads in and out of the city, establishing more checkpoints, launching raids against insurgent hideouts and calling in airstrikes if necessary.