PLO Chairman Yasser Arafat's security forces risked setting off a Palestinian civil war, arresting 112 followers of Hamas and Islamic Jihad today to retaliate forsuicide bombingsthat killed seven Israelis and an American college student Sunday. Militant Islamic leaders threatened resistance against Arafat's government: "If he (Arafat) practices this behavior, we will defend ourselves by all means," warned Mahmoud Zahar, a Hamas leader. The bombings, near Jewish settlements in the Gaza Strip, injured more than 40 people. Alisa Flatow, a 20-year-old junior at Brandeis University on vacation for Passover, died today of her wounds. Arafat's police chief, meanwhile, said the militant opposition to the Israel-PLO peace accord now amounts to just 150 gunmen who could easily be rounded up.