* Rally in Budapest biggest of its kind in recent years
* PM Orban scored third successive landslide victory
* Results reveal split between Budapest and rural areas
* Support for Orban's Fidesz lowest among younger people -survey
Tens of thousands of Hungarians protested in Budapest on Saturday against what organisers said was an unfair election system that gave Prime Minister Viktor Orban a landslide victory at the polls after a "hate campaign" against immigrants.
Orban won a third straight term in power in Sunday elections after his anti-immigration campaign message secured a strong majority for his ruling Fidesz party in parliament, giving him two-thirds of seats based on preliminary results.
In a Facebook post before the rally, organisers called for a recount of ballots, free media, a new election law, as well as more efficient cooperation among opposition parties instead of the bickering seen in the run-up to the vote.
The protest was among the biggest in Hungary in recent years, similar in size to a mass rally prompted by Orban's plan to tax internet use four years ago and a pro-government demonstration called by Orban supporters shortly before the election.
Fidesz received 49 percent of national party list votes and its candidates won 91 of 106 single-member constituencies, most of them in rural areas. Leftist opposition candidates carried two-thirds of the voting districts in Budapest.
There was a similar split between ages, with support for Orban's Fidesz at 37 percent among voters below 30, rising gradually to 46 percent among those older than 50, according to a survey by think tank Median published earlier this week.
The Organisation for Security and Cooperation in Europe has said the election did not offer opposition parties a level playing field amid a host of problems marring a vote that nonetheless generally respected fundamental rights.
In their Facebook post, the rally's organisers said: "Fidesz's election system and the government's hate campaign have pushed the majority into a one-third (parliamentary) minority."
Protesters marched from the Opera House, a 19th century Neo-Renaissance palace on a majestic downtown avenue, to Parliament by the Danube River, waving Hungary's tricolour flag and the European Union flag, accompanied by whistles and horns blaring.
As demonstrators filled a large square outside Parliament, many were still walking on main roads leading there, chanting: "We are the majority."
Organisers of the event estimated the size of the crowd at over 100,000 people, which briefly took on a festival-like atmosphere as a pop singer took the stage to perform an opening song blasting Orban's "pseudocracy."
"Viktor Orban has forgotten where he came from," Ors Lanyi, one of the organisers told the crowd, which responded with loud boos upon hearing the prime minister's name.
In contrast to Orban's closing rally last week in his native Szekesfehervar, where the overwhelming majority of supporters were middle-aged and elderly people, the Budapest protest attracted many people from younger generations.
"We are disappointed and I think lots of us are disappointed with the election results, which, I think, were not clean," said 26-year-old Palma, who declined to give her surname.
Palma, who came to the protest with a friend, said she believed the Hungarian election system gave an unfair advantage to Orban's ruling Fidesz party. However, she was also displeased with opposition parties.
"They are pathetic," she said. "It is terrible that they are so weak, unable to reach a compromise and they kill each other instead of joining forces for us."
Nationalist Jobbik and the Socialists are the biggest opposition groups in parliament.
In the election campaing Orban projected himself as the defender of Hungary's Christian culture against Muslim migration into Europe, an image which resonated with millions of voters, especially in rural areas.
But the opposition's poor showing was at least partly of its own making as rival candidates split the anti-government vote in five districts in Budapest, where preliminary results showed a slim Fidesz victory.
"Zero, zero, zero," 29-year-old Dia Szenasi said about the opposition, adding that all leftist parties should have joined forces to have a better chance of ousting Orban.