Libya's uprising reached the heart of Tripoli on Friday as anti-regime demonstrators defied a security clampdown to demand Muammar Gaddafi's overthrow amid hopes that key military units in the west of the country would defect.

Gunmen in cars reportedly opened fire on protesters as they streamed out of mosques after Friday prayers. Witnesses described shooting in streets near Green Square in the heart of the city.

Information remained patchy, confused and sometimes contradictory, but up to seven people were reported shot dead in Janzour, Fashlum, Bin Ashour, Zawiyat al-Dahmani and other urban areas. "Security forces fired indiscriminately on the demonstrators," said one resident.

Later, Gaddafi appeared in Green Square to give another angry and defiant speech to crowds of supporters waving banners and cheering him – a message that he is alive and in control – as he pledged to "open up the arsenals".

Libyan state TV, trying hard to maintain an air of normality in the virtual absence of independent media, denied the reports of deaths. Foreign journalists escorted into town from the airport by loyalists were confined to their hotels.

Ominously, there were claims by opposition sources that missing people were being held as human shields in the sprawling Bab al-Aziziya compound where Gaddafi lives and works.

Libyan exiles said that a reported rebellion by military personnel at Tripoli's Mitiga air base was linked to calls by air force officers in the liberated eastern city of Benghazi to come out against the regime. Analysts believe defections from the military are likely to prove more decisive than actual fighting as the nine-day uprising enters what may be its final phase.

The Libyan air force, which is dominated by members of the Megarha tribe, has traditionally been considered one of Gaddafi's most loyal supporters.

Army commanders in the east who have renounced Gaddafi's leadership have said that commanders in the west are now beginning to turn against him.

The loss of Mitiga would be a grave blow. The base, next to a civilian airport, is symbolically important as the former Wheelus base used by the US air force before the 1969 revolution. A correspondent for Italy's Ansa news agency reported that it was surrounded by troops and police.

Foreign diplomats monitoring Libyan developments said there were signs that more damaging high-level military and civilian defections may be imminent. Several key ministers and generals have abandoned Gaddafi in the last few days.

Fighting was also reported in al-Zawiya, 35 miles west of Tripoli, and Misurata, Libya's third largest city, to the east along the Mediterranean coast. Protesters were also said to be marching towards the capital from Tajura, an outlying suburb.

Witnesses spoke of pro-regime units equipped with tanks and automatic rifles, and wounded people being removed from hospitals to unknown destinations.

Gaddafi's son, Saif al-Islam, told foreign journalists that the regime was "dealing with terrorists" in Zawiya and Misurata who were running out of ammunition. "The army decided not to attack the terrorists, and to give a chance to negotiation," he said. "Hopefully we will do it peacefully and will do so by [today]."

The rebellion is strengthening its hold in the east but the balance of forces around the capital and in the west remains unclear. Three Berber towns in the mountains 90 miles south-west of Tripoli – Yefren, Zenten and Jadu – are in opposition hands, but other parts of the region are still under central control.

Gaddafi's position in Tripoli could depend in part on the performance of an elite unit led by one of his sons, Khamis, whose 32nd Brigade is one of three well-equipped regime protection units that total 10,000 men. The Libyan army, always a weak institution, has been hit badly by desertions, especially in the east. Libyan and Arab sources describe ongoing attempts to persuade key tribes to change sides, which are likely to affect the coherence of the army. According to unconfirmed reports there are divisions within the 32nd brigade.

The regime's continued defiance was reflected in attempts to persuade the international media that reports of its imminent demise have been exaggerated, especially by the influential Arabic satellite TV channel al-Jazeera.

Italian journalists were the first to be allowed in – a reflection of the close links between Libya and the former colonial power. But a BBC Arabic Service crew, ITN and other channels have now been given visas to enter the country. Other foreign correspondents are reporting from the Tunisian border or the Benghazi area, having entered Libya from Egypt.

Libyan opposition sources warned of the dangers of manipulation by the regime, complaining about a report on Thursday on the Saudi-owned al-Arabiya TV, which seemed credulous about official claims and failed to ask basic questions.

"It was a shameful and unprofessional performance that we saw from al-Arabiya," said one source. "They sent a young, inexperienced reporter who was guided around Tripoli and shown that everything was 'normal'. He was even introduced to some normal inhabitants of the Fashlum quarter of Tripoli, who confirmed the 'normality' of the situation and feigned their incredulity at the reports they hear from international news channels about their city.

"Such actions lend credence and support to a despot as he seeks to create enough confusion and doubt over what is happening in Tripoli, in order to provide him with a cover for his oppression of the Libyan people and the crimes against humanity he is visiting upon them."

More envoys quit



• Several Libyan diplomats resigned their posts or switched their allegiance to the opposition during the week.

At the UN in Geneva, diplomats declared they were defecting to the opposition Friday. At an emergency meeting of the UN Human Rights Council, a senior Libyan diplomat asked the council to stand for a moment of silence to "honor this revolution" and then informed the council the entire mission was quitting the government. Council members gave them a standing ovation.

In Cairo, the Libyan delegation to the Arab League abandoned Gaddafi and condemned his "heinous crimes against unarmed citizens".

In Paris, Tripoli's ambassador to France, Mohamed Salaheddine Zarem, and its ambassador to Unesco, Abdoulsalam el-Qallali, resigned on Friday.

In India, the entire staff of Libya's embassy in New Delhi renounced ties with Gaddafi's government. Ali al-Essawi, Libya's ambassador to India, resigned his post last weekend in protest at the violent crackdown.

Libyan ambassadors to Bangladesh, China, Indonesia, United States and Australia are also among those who no longer represent Gaddafi.