Pensions strike disrupts schools and ambulance service
More than 2,000 schools have been affected and services hit as public sector workers strike over pensions.
London Ambulance Service (LAS) said there was severe pressure on 999 services and called in police to help respond to emergency calls.
Up to 11,500 hospital workers also walked out across the city and there were pickets outside courts.
Refuse collections, day-care centres and library services were affected as councils ran reduced services.
In total, 2,092 London schools were closed or part-closed, 88% of the total number of schools in the capital.
Thousands marched in a protest rally of public sector workers starting from Holborn and finishing at Westminster.
Ahead of the march, two protesters were arrested after a police community support officer was assaulted in clashes surrounding strikes at a bus garage in Hackney, east London.
'Regrettable and wrong'
Scotland Yard also said it had arrested 77 people in connection with a variety of offences.
A total of 41 arrests were for breach of the peace in Dalston Lane, east London. Two of these people remain in custody, 39 have been bailed.
London Mayor Boris Johnson urged people to "go back to work", saying: "I think the strike is regrettable and wrong, it won't make a bean of difference to the outcome of the argument.
"I hope people, having made their point, will recognise there are kids who need to be educated, there's stuff that needs to be done in this city and they'll go back to work as soon as possible."
LAS said it had faced "severe pressure" after 42% of staff walked out, while NHS London strategic health authority said the service received 30% more 999 calls than normal.
Earlier on Wednesday the service said it was "struggling" and it had been very, very busy.
Throughout the day the service, which has trained 200 managers as emergency ambulance staff, prioritised the "most seriously ill and injured patients".
But on Wednesday evening it made a formal request to the Metropolitan Police Service (Met), City of London Police and British Transport Police for support in responding to 999 calls across London.
The Met said while the ambulance service dealt with "critical incidents" where lives were at risk, police provided medical support and transported medical staff where needed.
This assistance was no longer required as of 01:00 GMT.
No airport congestion
Staff formed picket lines outside the Old Bailey, the Royal Courts of Justice - which houses the High Court and Courts of Appeal - Westminster and West London Magistrates' Courts, the PCS union said.
Only one of the 10 court rooms at Westminster Magistrates' Court was open, leading to severe disruptions, but the murder trial of Stephen Lawrence at the Old Bailey was not affected as union leaders promised not to disrupt proceedings.
Unions said up to two million public sector workers took part in the UK-wide strike.
The unions object to the government's plans to make their members pay more and work longer to earn their pensions, but the government has accused union leaders of wanting to "wreck" the economic recovery.
On Tuesday, Liam Smith, the council leader at the Labour-run Barking and Dagenham Council, said in a statement he would not cross any picket lines.
BAA, which owns Heathrow Airport, had warned that passengers could face queues of up to three hours as a result of the strike by UK Border Agency immigration officers.
But on Wednesday, immigration controls were at two-thirds of normal staffing levels - more than the 30% to 50% predicted - and immigration queues were running at "normal levels".
The airport put out a plea to airlines last week to cut their passenger numbers.
Airlines managed to reduce the number of passengers by 25% by offering people a chance to change their flights without charge.
BAA said an extra 200 staff in purple uniforms had been on hand to help passengers, "offering information, food, drink and activity packs for children".
Hospital staff strike
Colin Matthews, BAA chief executive, said: "As a result of the whole airport community working together over the past few days we have more immigration officers on duty and fewer passengers arriving than would otherwise be the case.
"That has put us in a better place to avoid the serious delays and widespread disruption at Heathrow that were projected last week."
However Mr Matthews said they were expecting Thursday to be busier than usual and extra staff were on standby to help passengers.
Nur Alam, from the PCS union at Heathrow, said: "It was not our intention to affect members of the public, it was our intention to send a message to the government."
London City Airport, in east London, said the strike had had no impact.
And no delays were reported at St Pancras International Station, where the Eurostar comes in.
NHS London said 11,500 hospital workers had taken part in the action, although it had been feared many more would join the protest.
NHS London said there was "sufficient safe cover" at hospitals. Health workers set up pickets outside St Mary's Hospital and St Pancras Hospital, among others.
The British Museum was shut on Wednesday, but the Tate Modern, Tate Britain, the Science Museum and the Natural History Museum were open.
Chancellor George Osborne urged unions on Tuesday to "get back round the negotiating table" as he warned that the strike was "not going to achieve anything".
The government has said that with people living longer, the cost of public sector pensions is rising and reforms are needed.