At the Karelian Isthmus front there were on average 120 Red Army artillery pieces for every kilometer, with up to 220 artillery pieces per kilometer on the breakthrough sector at Battle of Valkeasaari.
The offensive opened with a huge aerial assault by the 1,600 strong 16th Air Army.
The Finnish Army was in a strong position behind fortified positions, but the Soviet air attacks undermined resistance and many Finnish units retreated and suffered from thousands of desertions. On June 9, the Soviet offensive began, surprising the defending Finnish army. During the day, the Soviets captured frontline trenches and destroyed fortifications, so they were in good starting positions when the main thrust began at the morning of June 10, which shattered the Finnish defence at the breakthrough sector. Already, on June 13, the VT-line was reached and although it held out in the Battle of Siiranmäki, the defensive position was breached at Battle of Kuuterselkä on June 15.
The Finnish army tried to buy time by fighting delaying actions when retreating so that additional forces from East Karelia would reach the front, and the VKT-line could be prepared for combat. However, on June 19 the first Leningrad Front forces had reached Vyborg, and the first phase of the offensive was completed by the capture of the city on June 20, when defending Finnish 20th Infantry Brigade fled in panic.
Mannerheim had asked for German help, and on June 17 Gefechtsverband Kuhlmey arrived in Finland, followed on June 21 by the 303rd Assault Gun Brigade (at half strength) and the 122nd Infantry Division. Also, new German anti-tank weapons, Panzerfausts and Panzerschrecks, were issued to Finnish army troops. On June 22, German foreign minister von Ribbentrop arrived to Finland in an attempt to extract political consessions from the military help. Also on June 22, Finnish government asked peace from Soviet Union, but when Soviet response arrived at the next day, it was interpreted as a demand for unconditional surrender.
On June 21, Stavka ordered continued attacks on the Imatra-Lappeenranta-Virojoki defence line, on the Salpaline sector of the front. Another group would attack northwards to Käkisalmi (now Priozersk, Russia) and surround the Finns defending the eastern VKT-line while preparations would be made for an advance towards Kotka, Kouvola and the Kymi river.
With Finnish army reinforcements, there were 268,000 Finnish army troops with 2,350 guns, 110 tanks/assault guns and 250 planes facing the two Red Army Fronts; 40% of the men and guns, and all the tanks were on the Isthmus. In all, the Red Army had a 6:5 advantage in men and 3-5:1 advantage in guns, planes and tanks against the Finnish army.
The offensive continued at June 25, when the Red Army breached the VKT-line at Tali, between the Bay of Vyborg and the Vuoksi river. At June 26 the Finnish president Ryti gave the guarantee to Ribbentrop that Finland would fight to the end alongside with Germany. In the ensuing battle, although the Leningrad Front managed to breach the VKT-line at Tali, but was bogged down at Ihantala, in the largest battle in Scandinavian history. When it became evident that a breakthrough was not possible at Ihantala, the Leningrad Front tried to double envelope the defenders with the twin assaults of the Bay of Vyborg and the Vuosalmi. However, the Finnish army was able to hold their positions on these sectors of the front.
At July 12 STAVKA ordered Leningrad front to release offensive elements from the Finnish front, and on July 15, the Red Army troops were ordered to assume a defensive posture, and offensive elements (mostly armour) were transferred to the German front for use in Operation Bagration.
The Finnish army had previously withdrawn most of its forces from the southern shore of the Svir River, so when the Red Army offensive started on 20 June, it did not achieve the desired surprise. The Karelian Front troops crossed the river the following day and secured a beachhead 8 km deep and 16 km wide. On June 23, a Northern Fleet naval infantry brigade attacked and captured a beachhead behind the Finnish lines between the Viteleenjoki and Tuuloksenjoki rivers, thus severing the main road along the shore of Lake Ladoga. Olonets was liberated on June 25, and on June 29, one of the main operation goals was achieved with the liberation of Petrozavodsk.
The Finnish army retreated further, delaying the Karelian Front advance, allowing for the U-line, running northwards from Pitkäranta to Loimola and Kivijärvi, to be reinforced. The first Karelian Front units reached the U-line on July 10, but were fatigued following the long offensive, and failed to breach the defence line.
The last attempt to resume the offensive was made further north, where two Soviet divisions advanced towards Ilomantsi. Their attack was initially successful, and these divisions reached the border of 1940 on July 21, (the only Soviet units who did so in the offensive), but during the ensuing battle these divisions were defeated and forced to retreat east.
The Vyborg and Svirsk-Petrozavodsk front-wide offensive operations lasted 61 days. As a result of the bitter fight, the Red Army liberated Karelia and the northern areas of the Leningrad region and defeated the Finnish Army. The city of Vyborg was taken by assault. It became part of the Russian territory. Success of the Soviet troops radically changed the situation in the northern part of the Soviet-German Front and predetermined the withdrawal of Finland from the war.