Authorities braced for a massive demonstration Wednesday in Bangkok that could pose a threat to Thailand's four-month-old government, and the prime minister vowed to crack down on any violence. Anti-government activists hoped to draw tens of thousands of supporters in the biggest demonstration since rival activists shut down the city's airports in November. Prime Minister Abhisit Vejjajiva, whose car was attacked and its window smashed by anti-government protesters Tuesday as he rode in it, called on all parties to keep the demonstration peaceful. He said his government would take any necessary measures to ensure they do so. "There are some groups which want violence so it's everyone's job to make sure it does not happen," Abhisit said. "I have tried to tell those who want to express themselves to rally peacefully."