Russia has launched a major "invasion" of Georgia and is now in control of half the country after a military operation by air-and-ground forces, Georgian military officials claimed this afternoon.

Georgia appealed urgently for international help and said it was planning an all-out defence of the capital, Tbilisi, from Russian attack. It said it had withdrawn its troops as Russian forces surged across the west and centre of the country.

Russian tanks and armoured vehicles seized the town of Gori - 36 miles from Tbilisi - at about 5pm local time, after advancing southwards this afternoon from the breakaway republic of South Ossetia, Georgian officials said, smashing the Georgian line in two places.

Tonight Saba Tsitsikashvili, a Gori resident and local Georgian journalist, said that Russian troops had occupied the main road on the edge of Gori. The troops were in the north of the city but had not yet moved towards the centre, he said.

He told the Guardian: "They are on the central highway which links the east and west of Georgia. This is the situation. People are leaving their villages. They have been told by policemen to leave their villages and houses.

"It's a very bad situation. People are in panic. Nobody knows what to do. This road where the troops are is about 2kms from the city centre. It's very near. The road is closed now."

At the same time Russian troops were moving across the west of the country from Georgia's second breakaway republic of Abkhazia, in what appeared to be a significant escalation in the five-day conflict.

Georgian military officials said Russian troops had seized the towns of Kutaisy and Senaki and were also planning to move forces to the port town of Porti.

"This is a classical full-scale invasion under the umbrella of peace enforcement," Irakli Batkuashvili, the head of Georgia's military planning division told the Guardian tonight.

He added grimly: "This is an occupation."

"Half of Georgia is under Russian control. Our aim now is to build up our troops and to create a defensive line in front of Tbilisi. We will fight defending Tbilisi."

The pace of the Russian attack appears to have caught Georgia's vastly inferior armed forces by surprise. Russia had previously insisted it had no intention of moving beyond the existing conflict zone in South Ossetia - briefly captured and then abandoned by Georgian forces.

Amid a day of claims and counter-claims, a senior Russian general had said there were no plans to move its troops deeper into Georgian territory. Russian officials denied that Gori had been occupied and Georgian assertions that it had fallen to Russian forces were disputed by some witnesses, who said they saw no troops in the deserted town.

The Georgian president, Mikhail Saaskashvili, visited Gori earlier in the day, before being bundled away by his security staff.

Georgia has asked for a ceasefire but Moscow has refused, arguing Georgian troops are still fighting in South Ossetia. Tblisi said up to 50 Russian bombers attacked its territory overnight, with one Russian bomb reported to have landed near the civilian airport of Tblisi, shortly after the French foreign minister, Bernard Kouchner, had touched down.

News of the latest fighting came as Britain condemned the Russian attacks as "deplorable". Gordon Brown called on Russia to accept Georgia's offer of a ceasefire, saying Russian military action threatened regional stability and could damage Moscow's ties with other countries.

"There is no justification for continued Russian military action in Georgia, which threatens the stability of the entire region and risks a humanitarian catastrophe," the prime minister said in a statement. "There is an immediate and pressing need to end the fighting and disengage all military forces in South Ossetia.

Nato's secretary general, Jaap de Hoop Scheffer, joined the US president, George Bush, in criticising Russia for "disproportionate" use of force. Nato said in April that Georgia would one day be a member of the alliance.

Bush told a US sports broadcaster in Beijing, where he was watching the Olympics, that he had spoken firmly to the Russian prime minister, Vladimir Putin. "We strongly condemn bombing outside of South Ossetia." Moscow has justified strengthening up forces in Abkhazia as an attempt to prevent what it called Georgia's "genocide" in South Ossetia's capital, Tskhinvali.

Putin in turn criticised the US for flying 2,000 Georgian soldiers back from Iraq, where they make up the third-biggest troop contributor after the US and Britain.

"It is a shame that some of our partners are not helping us but, essentially, are hindering us. I mean … the transfer by the United States of a Georgian contingent in Iraq with military transport planes practically to the conflict zone. The very scale of this cynicism is astonishing - the attempt to turn white into black, black into white and to adeptly portray victims of aggression as aggressors and place the responsibility for the consequences of the aggression on the victims."

Moscow says 1,600 civilians have been killed in South Ossetia, along with 15 Russian peacekeepers killed and 70 wounded. Saakashvili, told reporters at a press conference this afternoon Russia's attack against Georgia was "the pre-planned, cold-blooded murder of a small country''.

He accused Russia of attempting to occupy the whole country. "This provocation was aimed at occupying South Ossetia, Abkhazia and then all of Georgia."

Saakashvili said 90% of Georgian casualties were civilians. "They are specifically targeting civilian targets. They've been targeting highways in Georgia which are especially crowded at this time of year because of returning holidaymakers," he said.

Georgia had previously said 80% of casualties were military. Georgia's envoy to the EU, Salome Samadashvili, said the EU must tell Russia to pull back or " risk their future relationship with the European Union". EU foreign ministers are expected to meet over the crisis on Wednesday. There are concerns that oil prices, which have fallen in recent weeks, may rise again because of the crisis.

Saakashvili told the US vice-president, Dick Cheney, in a phone call that Russian aggression must not go unanswered. Cheney did not spell out what might follow, but threatening to expel Russia from the G8 is one possibility that has been raised in the past by the Republican presidential candidate, John McCain.

The Russian ambassador to Nato, Dmitry Rogozin, said Moscow had asked for a meeting with the alliance to explain its actions. Nato ambassadors are meeting tomorrow with Georgia's foreign minister, Ekaterina Tkeshelashvili.

The World Food Programme has started distributing food to help displaced people, and has said the number of people needing assistance is "rising by the hour''.