MORE than 4000 police officers will be deployed for Baroness Thatcher's funeral today amid heightened security after the Boston Marathon bombings.

Counter-terrorism chiefs have reviewed all measures, and the heavy police presence will mean officers will form a strong blue line along the funeral route from the Strand to St Paul's Cathedral.

Lord Reid of Cardowan, the former Labour home secretary, called for extra vigilance and said, in light of the US bombings, it was to be hoped "everyone recognises the added responsibility of co-operating with the police and the authorities" during today's event.

Lord Carrington, the foreign secretary in the 1979 Thatcher Government, said: "I hope there won't be too much trouble."

Christine Jones, the Metropolitan Police Commander, made clear that despite the security concerns, protests would be allowed.

It has been suggested some people might turn their backs on the coffin as it makes its way up Ludgate Hill, and that some might throw pieces of coal in remembrance of the miners' strike, or milk as a reminder of Lady Thatcher's nickname "milk snatcher" after she stopped free milk in schools.

Cdr Jones said: "We will continue to review our intelligence and evidence picture, and although to date no arrests connected to the ceremonial funeral have been made, should the need arise to arrest those who are committing acts of crime or violence, or conspiring to do so, we will respond accordingly."

With some 700 armed forces personnel lining the route and with representatives of those units most associated with the Falklands War carrying the coffin, it came as little surprise yesterday that No 10 confirmed Alicia Castro, the Argentinian ambassador to London, had turned down her invitation to attend. Asked why she had done so, Downing Street said that was a matter for the Argentine embassy.

Tensions with Buenos Aires have increased of late with the 30th anniversary of the conflict. President Cristina Fernandez de Kirchner has renewed her country's claims of sovereignty over the islands, accusing Britain of maintaining colonial rule.

Meanwhile, the White House confirmed that President Barack Obama would be sending two Reagan-era officials – former US secretaries of state James Baker and George Schultz. It was understood they were among a small group of guests invited to dinner last night in Downing Street by David Cameron.

His spokesman made clear the Prime Minister was not disappointed by the US official representation – which does not include any serving US politician – and insisted the absence of Mr Obama was "absolutely not" a snub.

He said the presence of two former US secretaries of state was testament to Lady Thatcher's "high global stature" and the esteem in which she was held internationally.

Among other figures who had accepted invitations to attend were Henry Kissinger, the former US secretary of state, and Dick Cheney, who was vice- president in the administration of George W Bush.

Earlier, family, friends and close political colleagues attended a private service at Westminster as Lady Thatcher returned to the scene of so many of her political battles for one last time.

The late PM's daughter Carol had tears in her eyes following the brief but emotional service in the crypt chapel of St Mary Undercroft. Around 150 guests, also including son Sir Mark Thatcher and several members of Lady Thatcher's government, filed past her coffin, many pausing to bow in respect to the UK's first female prime minister, who died last week at the age of 87.

Former Tory Party leader Iain Duncan Smith, the Work and Pensions Secretary, who was one of the most senior political figures to attend the ceremony, said: "It was very moving, short but moving, very sombre. It's different from tomorrow because this event was personal. Lots of people were there for personal reasons."

Earlier, the PM's spokesman said Mr Cameron would not attend the service, adding he was not aware of any plans for him to pay his respects in the chapel later.

"This afternoon's ceremony is one that is, very understandably, very much family-led. The right thing has been for the Prime Minister to lead tributes to Lady Thatcher in the House last week and to attend the funeral ceremony."

Tory MP Conor Burns, a close friend of Lady Thatcher who visited her almost every Sunday evening at her home, said the service was "beautiful".

On top of the coffin, draped in the Union flag, was a large bouquet of white roses and a hand-written card saying "Beloved Mother – Always in our Hearts".