The Europe-wide air traffic agency said it was optimistic the situation would be back to normal in a few days' time.
It also said more than 95,000 flights had been cancelled since last Thursday, a day after the volcano erupted for the second time in a month.
Weary passengers cheered and clapped as flights took off from airports such as Paris and Amsterdam, where flights resumed late on Monday.
"Everyone was screaming in the airplane from happiness," one passenger who flew from the Dutch capital to New York told the news agency AP.
Norway's airport authority reopened all of the country's airspace on Tuesday afternoon until midnight.
Elsewhere in Scandinavia, airports in north-central Sweden were operating, Denmark's airspace was open to long-haul flights, but Finland's was shut.
Germany's DFS air safety agency said its flight ban would remain until 0001 GMT on Wednesday, although 800 flights would be allowed to fly visually at lower altitudes, reports news agency AFP.
The UK's air traffic control authority, Nats, said on Tuesday afternoon that much of Britain would remain a no-fly zone until at least 0100 on Wednesday.
Only airspace in most of Scotland, Northern Ireland and parts of northern England will be open.
Nearly 300 British holiday-makers marooned in Santander, northern Spain, found a novel way to get home when they were picked up by a Royal Navy warship.
It's this shared experience of facing a common challenge that I'll remember - my faith in human kindness is renewed
Poland, which had reopened four airports on Monday, closed them again on Tuesday.
The Irish Aviation Authority said Shannon airport in the west was reopening, but the airports in Dublin and Cork remained shut.
Swiss and northern Italian airspace has also reopened. The Swiss authorities said test flights had shown the ash in the sky posed no threat to aircraft.
Flights have resumed out of Paris' Charles de Gaulle and Orly airports, which are operating at about 30% capacity.
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ABC reporter Neal Karlinksy: "The explosions... are silent"
In Spain, where all airports are open, the government has offered to let Britain and other European countries use its airports as stopovers to get passengers moving.
Our correspondent in Madrid, Sarah Rainsford, says that British passengers have come from from as as South Africa and Israel.
Financial impact
But she says there is still no sign there of the coaches the UK government promised it would send to help get its stranded nationals home.
In an effort to try to take control of the situation, EU transport ministers have created a core no-fly area, a limited-service zone and an open-skies area.
The EU Commissioner for Transport, Siim Kallas, has rejected criticism that the EU took too long to respond to the crisis.
Addressing the European Parliament in Strasbourg, Mr Kallas said the matter was not "in the hands of arbitrary decisions", as the lives of people were at stake.
The airline industry says its losses have soared to over $1bn (£650m; 740m euros), since much of Europe's airspace was closed last week because of volcano ash.
The flight ban was imposed because in the high temperatures of an engine turbine, ash can turn to molten glass and cripple the engine.
In a sign of the impact of the crisis on Asia's export-driven economies, the Japanese car giant, Nissan, says it is suspending several production lines due to the shortage of parts from Ireland. Honda will also partly halt production.
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