Tens of thousands of Thai protesters rode motorcycles and trucks Tuesday through cheering crowds in Bangkok, defying a police ban in a show of strength that may increase pressure on Prime Minister Abhisit Vejjajiva to call an election. Bystanders handed out water and boiled eggs to the red- shirted demonstrators as they occupied the commercial business district for a fourth day, seeking to provoke a police response that might bolster their campaign. An explosion outside the ruling party's headquarters injured two police officers, TNN television reported, without citing a source for the information. 'I am determined to enforce the law to maintain social order, but it will take time,' Abhisit said in a nationally televised address. 'Some of you may be unsatisfied. I urge your patience. The government will do our best to bring normalcy back to Thai society.' Abhisit said security forces exercised restraint 'to avoid confrontation' as tensions heightened after three weeks of mostly peaceful rallies. Broadening support within Bangkok would expand a largely rural movement that risks undermining a recovery from Thailand's first recession in a decade. 'The protesters' ability to rotate people in from the countryside and get help from people in Bangkok makes the likelihood they are going to quit and leave the city quite low,' said Michael Montesano, a visiting research fellow at Singapore's Institute of Southeast Asian Studies. 'The past week has made it undeniable that this is now a social movement.' Nurses, maids, restaurant workers, and executives in suits cheered on groups of protesters as they fanned out along 11 roads in downtown Bangkok that police had barred them from entering. Police and soldiers retreated after first attempting to block them. Force won't be used to disperse the crowd, Piya Uthayo, spokesman for the Metropolitan Police Bureau, said in a televised announcement after a scuffle. Thailand's Internal Security Act, in place since the rallies began, gives the military power to clear streets and make arrests. A year ago, soldiers quashed similar protests by Abhisit's opponents that started peacefully and turned violent. The prime minister said he may cancel a trip to the United States next week if the situation isn't resolved by April 10. The demonstrators, who say they represent lower classes of society, want an immediate election. They say the constitution written after the military ousted ex-leader Thaksin Shinawatra in a 2006 coup gives too much power to palace aides, judges, bureaucrats and soldiers at the expense of elected politicians. Since 1946, when King Bhumibol Adulyadej took the throne as an 18-year-old, the country has seen more than 20 prime ministers and nine coups. Only two of 17 Thai constitutions since absolute monarchy ended in 1932 have mandated fully elected parliaments. 'It's up to the government whether this will end violently,' said Krit Keerakiatwong, 42, owner of a Bangkok export business who has transformed his factory into a makeshift kitchen to provide meals for protesters. 'It's not easy for them to give away power, especially not to a group of mostly poor people.' Abhisit's opponents say he is illegitimate because he became prime minister in December 2008 after a court dissolved the pro-Thaksin ruling party for election fraud. They rejected his offer to call an election within nine months, demanding he step down within two weeks. Thaksin and his allies have won the past four elections on strong support in rural areas for his platform of cheap health care and village loans. The billionaire former prime minister has orchestrated protests from overseas since fleeing a Thai jail sentence in 2008. Abhisit has asserted his right to complete his term in office. He must call an election by the end of 2011. His Democrat party may win as many as 240 seats in a new ballot, or half the total, he said in a March 22 interview. The demonstrators plan to remain outside Bangkok's largest luxury shopping malls even if the court approves arrest warrants, Nattawut said Monday. Thailand's financial markets are closed Tuesday for a public holiday commemorating the founding of the Chakri Dynasty in 1782. With assistance from Anuchit Nguyen, Yumi Teso and Suttinee Yuvejwattana in Bangkok. 