* City centre is Misrata front line - Amnesty researcher
* Misrata lacks electricity, people get water from wells
* Rebel backer says insurgents celebrate battle gains
(Adds resident citing rebel gains, Libyan minister)
By Fredrik Dahl and Souhail Karam
TUNIS/RABAT, April 19 (Reuters) - Forces loyal to Muammar Gaddafi renewed the bombardment of Misrata on Tuesday, causing a number of casualties, an Amnesty International researcher in the besieged Libyan city said.
Despite the shelling, a resident who backs rebels fighting against better-equipped government troops said the insurgents made "big gains" and their enemies suffered significant losses. The claim could not be independently verified.
Libya's third-largest city, the insurgents' last major stronghold in the west of the North African country, has been under siege for more than seven weeks.
Rebels and residents say pro-Gaddafi forces have pounded Misrata heavily in recent days, firing rockets and mortars at insurgent positions and also hitting residential areas.
"They were shelling very close by, in the area slightly to the northwest of the centre. I just left the hospital, there were casualties coming in," Amnesty researcher Donatella Rovera, who came to Misrata late last week, said by phone.
"These are the areas which are, for now, in the hands of the opposition and they are being shelled by Gaddafi forces," she told Reuters. "The city centre is the front line."
Rebel spokesmen, citing hospital records, say hundreds of people have been killed in Misrata during the siege.
Aid groups say conditions are worsening in the city of 300,000, with a lack of food, medicines and other basic items.
International humanitarian organisations have started evacuating trapped civilians by boat from its rebel-held port.
"There is no electricity. The town is functioning on generators," Rovera said. "The supply of water has now been cut off for weeks. They've gone back to using old wells."
In Geneva, the U.N. Children's Fund (UNICEF) said at least 20 children have been killed in weeks of fighting in Misrata.
Libyan officials say they are fighting militia with ties to al Qaeda bent on destroying the country, and deny government troops are shelling Misrata and its civilians.
"There are just pockets of violence attacking the armed forces from time to time and then the armed forces have to respond to those attacks,"Deputy Foreign Minister Khaled Kaim told reporters in Tripoli.
The Misrata resident said five rebels were killed in the day's fighting but the pro-Gaddafi forces "suffered heavy losses", both in the number of dead and those who surrendered.
"There were big gains for the rebels today," he said. "The rebels celebrated their gains today by driving cars through the city, waving their guns out of the windows and honking."
Unlike eastern Libya, where rebels control many coastal cities, most of the west remains firmly under Gaddafi's control.
A rebel spokesman in the western city of Zawiyah, which insurgents held for several weeks after the uprising against Gaddafi erupted in mid-February, said they were now using guerrilla tactics there.
"Groups, each comprising between 15 and 20 rebels, keep ambushing Gaddafi forces ... We have managed to kill dozens of them," the rebel who called himself Mohamed said by phone.
"Since they took control of the city, they have arrested thousands, mostly young men on the slightest suspicion of sympathy for the rebels," Mohamed said. (Additional reporting by Mussab Al-Khairalla in Tripoli; Joseph Nasr in Berlin and Marie-Louise Gumuchian in Tunis; editing by Tim Pearce)